diff --git a/lib/unsafe.js b/lib/unsafe.js
index c5c5d4a..5a4e0e5 100644
--- a/lib/unsafe.js
+++ b/lib/unsafe.js
@@ -130,7 +130,18 @@ export const unsafe = [
   // Caret is not used in markdown for constructs.
   // An underscore can start emphasis, strong, or a thematic break.
   {atBreak: true, character: '_'},
-  {character: '_', inConstruct: 'phrasing', notInConstruct: fullPhrasingSpans},
+  {
+    character: '_',
+    before: '[\\s]',
+    inConstruct: 'phrasing',
+    notInConstruct: fullPhrasingSpans
+  },
+  {
+    character: '_',
+    after: '[\\s]',
+    inConstruct: 'phrasing',
+    notInConstruct: fullPhrasingSpans
+  },
   // A grave accent can start code (fenced or text), or it can break out of
   // a grave accent code fence.
   {atBreak: true, character: '`'},
diff --git a/test/index.js b/test/index.js
index e47facd..a182b1e 100644
--- a/test/index.js
+++ b/test/index.js
@@ -3895,6 +3895,39 @@ test('escape', async function (t) {
     }
   )
 
+  await t.test('should escape underscore at word start', async function () {
+    assert.equal(
+      to({
+        type: 'paragraph',
+        children: [{type: 'text', value: '_WORLD'}]
+      }),
+      '\\_WORLD\n'
+    )
+  })
+
+  await t.test('should escape underscore at word end', async function () {
+    assert.equal(
+      to({
+        type: 'paragraph',
+        children: [{type: 'text', value: 'HELLO_'}]
+      }),
+      'HELLO\\_\n'
+    )
+  })
+
+  await t.test(
+    'should not escape underscore inside of word',
+    async function () {
+      assert.equal(
+        to({
+          type: 'paragraph',
+          children: [{type: 'text', value: 'HELLO_WORLD'}]
+        }),
+        'HELLO_WORLD\n'
+      )
+    }
+  )
+
   await t.test(
     'should escape what would otherwise be a heading (atx)',
     async function () {
@@ -4503,6 +4536,40 @@ test('roundtrip', async function (t) {
     )
   })
 
+  await t.test(
+    'should roundtrip underscore without escaping',
+    async function () {
+      const value = `Separate paragraphs:
+
+:heavy_check_mark:
+
+[HELLO_WORLD.md](HELLO_WORLD.md)
+
+HELLO_WORLD https://some/web_site
+
+HELLO_WORLD. HELLO_WORLD, H_W; HELLO_OTHER! HELLO_YES? HELLO_NO
+
+HELLO_WORLD.HELLO_WORLD,H_W;HELLO_OTHER!HELLO_YES?HELLO_NO
+
+HELLO_WORLD - THIS_GOOD
+
+HELLO_WORLD-THIS_GOOD
+
+One Paragraph:
+
+:heavy_check_mark:
+[HELLO_WORLD.md](HELLO_WORLD.md)
+HELLO_WORLD https://some/web_site
+HELLO_WORLD. HELLO_WORLD, H_W; HELLO_OTHER! HELLO_YES? HELLO_NO
+HELLO_WORLD.HELLO_WORLD,H_W;HELLO_OTHER!HELLO_YES?HELLO_NO
+HELLO_WORLD - THIS_GOOD
+HELLO_WORLD-THIS_GOOD
+`
+
+      assert.equal(to(from(value)), value)
+    }
+  )
+
   await t.test(
     'should roundtrip a sole blank line in fenced code',
     async function () {
@@ -4781,6 +4848,20 @@ a __\\_ is this emphasis? __\\_
 
 a _\\__ is this emphasis? _\\__
 
+a _ is_this emphasis? _
+
+a __ is_this emphasis? __
+
+a ___ is_this emphasis? ___
+
+a _\\_ is_this emphasis? _\\_
+
+a \\__ is_this emphasis? \\__
+
+a __\\_ is_this emphasis? __\\_
+
+a _\\__ is_this emphasis? _\\__
+
 One paragraph:
 
 a _ is this emphasis? _
@@ -4789,7 +4870,15 @@ a ___ is this emphasis? ___
 a _\\_ is this emphasis? _\\_
 a \\__ is this emphasis? \\__
 a __\\_ is this emphasis? __\\_
-a _\\__ is this emphasis? _\\__`
+a _\\__ is this emphasis? _\\__
+
+a _ is_this emphasis? _
+a __ is_this emphasis? __
+a ___ is_this emphasis? ___
+a _\\_ is_this emphasis? _\\_
+a \\__ is_this emphasis? \\__
+a __\\_ is_this emphasis? __\\_
+a _\\__ is_this emphasis? _\\__`
     const tree = from(value)
 
     assert.deepEqual(