diff --git a/README.md b/README.md
index 676c448..d9ccee2 100644
--- a/README.md
+++ b/README.md
@@ -18,6 +18,7 @@ xhtmlmd is largely implemented using AI, except for the tests. The tests are lar
 - Definition lists: PHP Markdown Extra/Pandoc-style `Term` followed by `: definition` or `~ definition`.
 - Footnotes: `[^id]` references to defined `[^id]:` definitions with indented continuation blocks.
 - Abbreviations: `*[HTML]: Hyper Text Markup Language` definitions render matching text as `<abbr>`.
+- Underline (opt-in): with `underline=True`, Discord-style `__x__` renders as `<u>x</u>` while `**x**` stays `<strong>`.
 - Fenced divs: Pandoc/Quarto/Djot-style `:::` containers with attributes or a single class word.
 
 ## Usage
@@ -54,7 +55,7 @@ Python callers can override rendered nodes with callbacks. Each callback receive
 Callback names:
 
 - Blocks: `paragraph`, `heading`, `block_quote`, `list`, `definition_list`, `code_block`, `html_block`, `html_container`, `thematic_break`, `table`, `div`, `math_block`
-- Inlines: `text`, `soft_break`, `hard_break`, `emph`, `strong`, `strike`, `superscript`, `subscript`, `highlight`, `code`, `link`, `image`, `autolink`, `abbr`, `html_inline`, `math_inline`, `footnote_ref`, `span`
+- Inlines: `text`, `soft_break`, `hard_break`, `emph`, `strong`, `underline`, `strike`, `superscript`, `subscript`, `highlight`, `code`, `link`, `image`, `autolink`, `abbr`, `html_inline`, `math_inline`, `footnote_ref`, `span`
 
 ```python
 from fastpylight import highlight
diff --git a/docs/DIALECT.md b/docs/DIALECT.md
index d5501aa..cb5e8b2 100644
--- a/docs/DIALECT.md
+++ b/docs/DIALECT.md
@@ -8,6 +8,7 @@ When Markdown extensions disagree, this crate chooses the behavior closest to Pa
 - Definition lists follow PHP Markdown Extra/Pandoc: one-line terms with one or more `:` or `~` definitions.
 - Footnotes follow Pandoc/kramdown label rules and render as XHTML endnotes with backlinks. The endnotes `<section>` has no leading `<hr>` (unlike cmark-gfm): separators are a styling concern, so add one with CSS if wanted.
 - Inline `~~x~~` renders as strikethrough. Inline `~x~` renders as subscript, using the same no-whitespace rule as superscript `^x^`.
+- The opt-in `underline` option follows Discord: `__x__` renders as `<u>x</u>` instead of `<strong>x</strong>`, with the usual underscore emphasis rules (no intraword, `___x___` nests as `<em><u>x</u></em>`). Off by default because CommonMark defines `__x__` as strong.
 - `<tag markdown="1">` parses block Markdown inside the balanced tag. `markdown="span"` parses inline content into a single paragraph child.
 - Raw HTML passes through unbalanced, per CommonMark. The opt-in `balance` option closes unclosed elements at the fragment end, drops stray closes, and self-closes void tags, without HTML5 implied-end-tag rules.
 - Math defaults to `MathMode::Brackets`, which recognizes `\(...\)`, `\[...\]`, and `$$...$$`. `MathMode::Dollars` also recognizes `$...$` with Pandoc's guard against currency-like spans. `MathMode::On` preserves backslashes before `[]()` so client-side renderers such as KaTeX can see TeX delimiters. `MathMode::Off` treats TeX delimiters as ordinary Markdown text.
diff --git a/python/xhtmlmd/__init__.py b/python/xhtmlmd/__init__.py
index f8eb135..557410c 100644
--- a/python/xhtmlmd/__init__.py
+++ b/python/xhtmlmd/__init__.py
@@ -3,11 +3,12 @@
 __all__ = ["to_xhtml", "render", "blocks"]
 
 
-def to_xhtml(markdown: str, *, math: str = "brackets", tagfilter: bool = False, balance: bool = False, callbacks: dict | None = None,
+def to_xhtml(markdown: str, *, math: str = "brackets", tagfilter: bool = False, balance: bool = False, underline: bool = False,
+    callbacks: dict | None = None,
     max_inline_depth: int | None = None, max_block_depth: int | None = None, max_link_paren_depth: int | None = None) -> str:
     "Render Markdown to an XHTML fragment."
-    return _to_xhtml(markdown, math=math, tagfilter=tagfilter, balance=balance, callbacks=callbacks, max_inline_depth=max_inline_depth,
-        max_block_depth=max_block_depth, max_link_paren_depth=max_link_paren_depth)
+    return _to_xhtml(markdown, math=math, tagfilter=tagfilter, balance=balance, underline=underline, callbacks=callbacks,
+        max_inline_depth=max_inline_depth, max_block_depth=max_block_depth, max_link_paren_depth=max_link_paren_depth)
 
 
 render = to_xhtml
diff --git a/python/xhtmlmd/__main__.py b/python/xhtmlmd/__main__.py
index dac345f..12c24fa 100644
--- a/python/xhtmlmd/__main__.py
+++ b/python/xhtmlmd/__main__.py
@@ -3,20 +3,22 @@
 
 from . import to_xhtml
 
-USAGE = ("usage: xhtmlmd [--math=off|on|brackets|dollars] [--balance] [file.md]\n\n"
+USAGE = ("usage: xhtmlmd [--math=off|on|brackets|dollars] [--balance] [--underline] [file.md]\n\n"
     "Reads Markdown from a file or stdin and writes XHTML fragment output. Math defaults to brackets.\n"
-    "--balance closes unclosed raw HTML tags and drops stray closing tags.")
+    "--balance closes unclosed raw HTML tags and drops stray closing tags.\n"
+    "--underline renders Discord-style __x__ as <u>x</u> instead of <strong>x</strong>.")
 
 def main(argv=None):
     argv = sys.argv[1:] if argv is None else argv
-    math, balance, file = "brackets", False, None
+    math, balance, underline, file = "brackets", False, False, None
     for arg in argv:
         if arg in ("--math=off", "--math=on", "--math=brackets", "--math=dollars"): math = arg.split("=", 1)[1]
         elif arg == "--balance": balance = True
+        elif arg == "--underline": underline = True
         elif arg in ("-h", "--help"): print(USAGE); return
         elif arg.startswith("--"): print(f"unknown option: {arg}", file=sys.stderr); sys.exit(2)
         else: file = arg
     src = open(file, encoding="utf-8").read() if file else sys.stdin.read()
-    sys.stdout.write(to_xhtml(src, math=math, balance=balance))
+    sys.stdout.write(to_xhtml(src, math=math, balance=balance, underline=underline))
 
 if __name__ == "__main__": main()
diff --git a/src/ast.rs b/src/ast.rs
index d04a147..74290fb 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -222,6 +222,10 @@ pub enum Inline {
         attrs: Attr,
         children: Vec<Inline>,
     },
+    Underline {
+        attrs: Attr,
+        children: Vec<Inline>,
+    },
     Strike {
         attrs: Attr,
         children: Vec<Inline>,
@@ -283,6 +287,7 @@ impl Inline {
         match self {
             Inline::Emph { attrs, .. }
             | Inline::Strong { attrs, .. }
+            | Inline::Underline { attrs, .. }
             | Inline::Strike { attrs, .. }
             | Inline::Superscript { attrs, .. }
             | Inline::Subscript { attrs, .. }
diff --git a/src/inline.rs b/src/inline.rs
index 3c822b7..19888a9 100644
--- a/src/inline.rs
+++ b/src/inline.rs
@@ -247,7 +247,7 @@ fn parse_inner(src: &str, ctx: &InlineContext<'_>, depth: usize) -> Vec<Inline>
         }
     }
     scanner.flush_text();
-    process_delimiters(&mut nodes, &mut delimiters);
+    process_delimiters(&mut nodes, &mut delimiters, ctx.options.underline);
     nodes_to_inlines(&nodes)
 }
 
@@ -412,7 +412,13 @@ impl InlineScanner<'_, '_> {
                 return None;
             }
         };
-        process_delimiters_range(self.nodes, self.delimiters, opener.node + 1, target_end);
+        process_delimiters_range(
+            self.nodes,
+            self.delimiters,
+            opener.node + 1,
+            target_end,
+            self.ctx.options.underline,
+        );
         let children = collect_node_inlines(self.nodes, opener.node + 1, target_end);
         match &mut item {
             Inline::Link { children: dst, .. } | Inline::Span { children: dst, .. } => {
@@ -617,8 +623,8 @@ fn delimiter_run_flags(ch: char, before: char, after: char) -> (bool, bool) {
     }
 }
 
-fn process_delimiters(nodes: &mut [Node], delimiters: &mut [Delimiter]) {
-    process_delimiters_range(nodes, delimiters, 0, usize::MAX);
+fn process_delimiters(nodes: &mut [Node], delimiters: &mut [Delimiter], underline: bool) {
+    process_delimiters_range(nodes, delimiters, 0, usize::MAX, underline);
 }
 
 fn process_delimiters_range(
@@ -626,6 +632,7 @@ fn process_delimiters_range(
     delimiters: &mut [Delimiter],
     start_node: usize,
     end_node: usize,
+    underline: bool,
 ) {
     // cmark's openers_bottom: when no opener matches a closer, remember how far
     // the search went per (char, can_open, len % 3) so later closers of the same
@@ -653,7 +660,7 @@ fn process_delimiters_range(
             closer += 1;
             continue;
         };
-        if wrap_delimiters(nodes, delimiters, opener, closer, use_len) {
+        if wrap_delimiters(nodes, delimiters, opener, closer, use_len, underline) {
             if delimiters[closer].len == 0 {
                 closer += 1;
             }
@@ -720,6 +727,7 @@ fn wrap_delimiters(
     opener: usize,
     closer: usize,
     use_len: usize,
+    underline: bool,
 ) -> bool {
     let open_node = delimiters[opener].node;
     let close_node = delimiters[closer].node;
@@ -738,6 +746,10 @@ fn wrap_delimiters(
             attrs: Attr::default(),
             children,
         },
+        '_' if use_len == 2 && underline => Inline::Underline {
+            attrs: Attr::default(),
+            children,
+        },
         _ if use_len == 2 => Inline::Strong {
             attrs: Attr::default(),
             children,
@@ -1754,6 +1766,13 @@ fn normalize_inline(item: Inline) -> Inline {
             }
             Inline::Strong { attrs, children }
         }
+        Inline::Underline { attrs, children } => {
+            let mut children = coalesce(children);
+            if attrs.is_empty() {
+                children = flatten_empty_underline(children);
+            }
+            Inline::Underline { attrs, children }
+        }
         Inline::Strike { attrs, children } => Inline::Strike {
             attrs,
             children: coalesce(children),
@@ -1806,3 +1825,18 @@ fn flatten_empty_strong(children: Vec<Inline>) -> Vec<Inline> {
     }
     out
 }
+
+fn flatten_empty_underline(children: Vec<Inline>) -> Vec<Inline> {
+    let mut out = Vec::with_capacity(children.len());
+    for child in children {
+        match child {
+            Inline::Underline { attrs, children } if attrs.is_empty() => {
+                for grandchild in children {
+                    push_coalesced(&mut out, grandchild);
+                }
+            }
+            x => push_coalesced(&mut out, x),
+        }
+    }
+    out
+}
diff --git a/src/lib.rs b/src/lib.rs
index 6bb3fb1..6f02fec 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -36,6 +36,7 @@ pub struct Options {
     pub math: MathMode,
     pub tagfilter: bool,
     pub balance: bool,
+    pub underline: bool,
     pub max_inline_depth: usize,
     pub max_block_depth: usize,
     pub max_link_paren_depth: usize,
@@ -47,6 +48,7 @@ impl Default for Options {
             math: MathMode::Brackets,
             tagfilter: false,
             balance: false,
+            underline: false,
             max_inline_depth: 64,
             max_block_depth: 128,
             max_link_paren_depth: 32,
diff --git a/src/python.rs b/src/python.rs
index 945291e..6149c2e 100644
--- a/src/python.rs
+++ b/src/python.rs
@@ -15,6 +15,7 @@ use crate::{MathMode, Options};
     math = "brackets",
     tagfilter = false,
     balance = false,
+    underline = false,
     callbacks = None,
     max_inline_depth = None,
     max_block_depth = None,
@@ -25,6 +26,7 @@ fn to_xhtml(
     math: &str,
     tagfilter: bool,
     balance: bool,
+    underline: bool,
     callbacks: Option<Bound<'_, PyDict>>,
     max_inline_depth: Option<usize>,
     max_block_depth: Option<usize>,
@@ -34,6 +36,7 @@ fn to_xhtml(
         math: parse_math_mode(math)?,
         tagfilter,
         balance,
+        underline,
         ..Options::default()
     };
     if let Some(depth) = max_inline_depth {
@@ -194,6 +197,7 @@ fn transform_inline(item: &mut Inline, callbacks: &Bound<'_, PyDict>) -> PyResul
     match item {
         Inline::Emph { children, .. }
         | Inline::Strong { children, .. }
+        | Inline::Underline { children, .. }
         | Inline::Strike { children, .. }
         | Inline::Highlight { children, .. }
         | Inline::Link { children, .. }
@@ -276,6 +280,7 @@ fn inline_kind(item: &Inline) -> &'static str {
         Inline::HardBreak => "hard_break",
         Inline::Emph { .. } => "emph",
         Inline::Strong { .. } => "strong",
+        Inline::Underline { .. } => "underline",
         Inline::Strike { .. } => "strike",
         Inline::Superscript { .. } => "superscript",
         Inline::Subscript { .. } => "subscript",
@@ -401,6 +406,7 @@ fn inline_node<'py>(py: Python<'py>, item: &Inline) -> PyResult<Bound<'py, PyDic
         Inline::SoftBreak | Inline::HardBreak => {}
         Inline::Emph { attrs, children }
         | Inline::Strong { attrs, children }
+        | Inline::Underline { attrs, children }
         | Inline::Strike { attrs, children }
         | Inline::Highlight { attrs, children }
         | Inline::Span { attrs, children } => {
diff --git a/src/render.rs b/src/render.rs
index 721e15b..92a33db 100644
--- a/src/render.rs
+++ b/src/render.rs
@@ -332,6 +332,13 @@ impl<'a> Renderer<'a> {
                 self.inlines(children, out);
                 out.push_str("</strong>");
             }
+            Inline::Underline { attrs, children } => {
+                out.push_str("<u");
+                attrs_html(attrs, out);
+                out.push('>');
+                self.inlines(children, out);
+                out.push_str("</u>");
+            }
             Inline::Strike { attrs, children } => {
                 out.push_str("<del");
                 attrs_html(attrs, out);
@@ -603,6 +610,7 @@ pub(crate) fn plain(items: &[Inline]) -> String {
             Inline::SoftBreak | Inline::HardBreak => out.push(' '),
             Inline::Emph { children, .. }
             | Inline::Strong { children, .. }
+            | Inline::Underline { children, .. }
             | Inline::Strike { children, .. }
             | Inline::Highlight { children, .. }
             | Inline::Span { children, .. }
diff --git a/tests/test_focused.py b/tests/test_focused.py
index c0bca57..7fec1da 100644
--- a/tests/test_focused.py
+++ b/tests/test_focused.py
@@ -90,6 +90,22 @@ def test_balance_ignores_rawtext_and_voids():
     assert "</div>" not in html
     assert "<br />" in html
 
+def test_underline_is_opt_in():
+    assert to_xhtml("__x__") == "<p><strong>x</strong></p>\n"
+    assert to_xhtml("__x__", underline=True) == "<p><u>x</u></p>\n"
+    assert to_xhtml("**x** _y_ __z__", underline=True) == "<p><strong>x</strong> <em>y</em> <u>z</u></p>\n"
+
+def test_underline_follows_underscore_emphasis_rules():
+    assert to_xhtml("___x___", underline=True) == "<p><em><u>x</u></em></p>\n"
+    assert to_xhtml("____x____", underline=True) == "<p><u>x</u></p>\n"
+    assert to_xhtml("intra__word__", underline=True) == "<p>intra__word__</p>\n"
+    assert to_xhtml("__a **b** c__", underline=True) == "<p><u>a <strong>b</strong> c</u></p>\n"
+
+def test_underline_callback():
+    cb = lambda node, default: default.replace("<u>", '<u class="d">') if node["type"] == "underline" else None
+    html = to_xhtml("__x__", underline=True, callbacks={"underline": cb})
+    assert html == '<p><u class="d">x</u></p>\n'
+
 def test_long_nonascii_words_near_autolink_cap_do_not_error():
     for boundary in ("(", "a: ", "x '"):
         for count in (126, 127, 128, 129, 130, 200):