diff --git a/src/components/ancestor-lines.tsx b/src/components/ancestor-lines.tsx
index ff4ea2d..a94b975 100644
--- a/src/components/ancestor-lines.tsx
+++ b/src/components/ancestor-lines.tsx
@@ -40,6 +40,7 @@ export function AncestorLines({ ancestors, onToggle, lineWidth }: AncestorLinesP
             onToggle={() => onToggle(ancestor.uri)}
             authorName={ancestor.authorName}
             replyCount={ancestor.replyCount}
+            ancestorUri={ancestor.uri}
             opacity={opacity}
             showChevron={false}
             width={lineWidth}
diff --git a/src/components/reply-branch.tsx b/src/components/reply-branch.tsx
index 6cf0353..0e6ef91 100644
--- a/src/components/reply-branch.tsx
+++ b/src/components/reply-branch.tsx
@@ -147,6 +147,7 @@ export function ReplyBranch({
                   onToggle={() => toggleCollapse(node.reply.uri)}
                   authorName={authorName}
                   replyCount={descendantCount}
+                  ancestorUri={node.reply.uri}
                   opacity={1}
                   showChevron={showChevron}
                   width={indentStep}
diff --git a/src/components/reply-thread.tsx b/src/components/reply-thread.tsx
index 13f2cab..49ce0cb 100644
--- a/src/components/reply-thread.tsx
+++ b/src/components/reply-thread.tsx
@@ -13,6 +13,7 @@ import type { Reply } from '@/lib/api/types'
 import { cn } from '@/lib/utils'
 import { buildReplyTree, flattenReplyTree } from '@/lib/build-reply-tree'
 import { useThreadIndent } from '@/hooks/use-thread-indent'
+import { ThreadHoverProvider } from '@/context/thread-hover-context'
 import { ReplyBranch } from './reply-branch'
 
 interface ReplyThreadProps {
@@ -58,17 +59,19 @@ export function ReplyThread({
           <p className="text-muted-foreground">No replies yet. Be the first to respond!</p>
         </div>
       ) : (
-        <ReplyBranch
-          nodes={tree}
-          postNumberMap={postNumberMap}
-          topicUri={topicUri}
-          allReplies={allReplies}
-          indentStep={indentStep}
-          showChevron={showChevron}
-          onReply={onReply}
-          onDeleteReply={onDeleteReply}
-          currentUserDid={currentUserDid}
-        />
+        <ThreadHoverProvider>
+          <ReplyBranch
+            nodes={tree}
+            postNumberMap={postNumberMap}
+            topicUri={topicUri}
+            allReplies={allReplies}
+            indentStep={indentStep}
+            showChevron={showChevron}
+            onReply={onReply}
+            onDeleteReply={onDeleteReply}
+            currentUserDid={currentUserDid}
+          />
+        </ThreadHoverProvider>
       )}
     </section>
   )
diff --git a/src/components/thread-line.test.tsx b/src/components/thread-line.test.tsx
index 9840b6d..e1c876d 100644
--- a/src/components/thread-line.test.tsx
+++ b/src/components/thread-line.test.tsx
@@ -2,33 +2,43 @@ import { describe, it, expect, vi } from 'vitest'
 import { render, screen } from '@testing-library/react'
 import userEvent from '@testing-library/user-event'
 import { axe } from 'vitest-axe'
+import { ThreadHoverProvider } from '@/context/thread-hover-context'
 import { ThreadLine } from './thread-line'
 
+function renderWithProvider(ui: React.ReactElement) {
+  return render(<ThreadHoverProvider>{ui}</ThreadHoverProvider>)
+}
+
 describe('ThreadLine', () => {
   const defaultProps = {
     expanded: true,
     onToggle: vi.fn(),
     authorName: 'Alex',
     replyCount: 5,
+    ancestorUri: 'at://test/line1',
     opacity: 1,
     showChevron: true,
   }
 
   it('renders as a button', () => {
-    render(<ThreadLine {...defaultProps} />)
+    renderWithProvider(<ThreadLine {...defaultProps} />)
     expect(screen.getByRole('button')).toBeInTheDocument()
   })
 
   it('has aria-expanded matching expanded prop', () => {
-    const { rerender } = render(<ThreadLine {...defaultProps} expanded={true} />)
+    const { rerender } = renderWithProvider(<ThreadLine {...defaultProps} expanded={true} />)
     expect(screen.getByRole('button')).toHaveAttribute('aria-expanded', 'true')
 
-    rerender(<ThreadLine {...defaultProps} expanded={false} />)
+    rerender(
+      <ThreadHoverProvider>
+        <ThreadLine {...defaultProps} expanded={false} />
+      </ThreadHoverProvider>
+    )
     expect(screen.getByRole('button')).toHaveAttribute('aria-expanded', 'false')
   })
 
   it('has descriptive aria-label including author and reply count', () => {
-    render(<ThreadLine {...defaultProps} expanded={true} replyCount={5} />)
+    renderWithProvider(<ThreadLine {...defaultProps} expanded={true} replyCount={5} />)
     const label = screen.getByRole('button').getAttribute('aria-label')!
     expect(label).toContain('Alex')
     expect(label).toContain('5')
@@ -37,7 +47,7 @@ describe('ThreadLine', () => {
   it('calls onToggle when clicked', async () => {
     const user = userEvent.setup()
     const onToggle = vi.fn()
-    render(<ThreadLine {...defaultProps} onToggle={onToggle} />)
+    renderWithProvider(<ThreadLine {...defaultProps} onToggle={onToggle} />)
     await user.click(screen.getByRole('button'))
     expect(onToggle).toHaveBeenCalledTimes(1)
   })
@@ -50,25 +60,25 @@ describe('ThreadLine', () => {
   })
 
   it('does not render chevron icon when showChevron is false', () => {
-    const { container } = render(<ThreadLine {...defaultProps} showChevron={false} />)
+    const { container } = renderWithProvider(<ThreadLine {...defaultProps} showChevron={false} />)
     expect(container.querySelector('svg')).not.toBeInTheDocument()
   })
 
   it('applies opacity style to the line', () => {
-    const { container } = render(<ThreadLine {...defaultProps} opacity={0.5} />)
+    const { container } = renderWithProvider(<ThreadLine {...defaultProps} opacity={0.5} />)
     const line = container.querySelector('[aria-hidden="true"]')
     expect(line).toBeInTheDocument()
     expect(line!.getAttribute('style')).toContain('opacity')
   })
 
   it('applies width from prop', () => {
-    const { container } = render(<ThreadLine {...defaultProps} width={22} />)
+    const { container } = renderWithProvider(<ThreadLine {...defaultProps} width={22} />)
     const button = container.querySelector('button')!
     expect(button.style.width).toBe('22px')
   })
 
   it('passes axe accessibility check', async () => {
-    const { container } = render(<ThreadLine {...defaultProps} />)
+    const { container } = renderWithProvider(<ThreadLine {...defaultProps} />)
     const results = await axe(container)
     expect(results).toHaveNoViolations()
   })
diff --git a/src/components/thread-line.tsx b/src/components/thread-line.tsx
index 3499a4b..f4a0d84 100644
--- a/src/components/thread-line.tsx
+++ b/src/components/thread-line.tsx
@@ -1,17 +1,21 @@
 /**
  * ThreadLine - Clickable vertical line with chevron for collapsing thread branches.
- * Visual width: 2px. Tap target: 44px minimum for accessibility.
  * Chevron: CaretDown (expanded) / CaretRight (collapsed). Hidden on mobile.
  * Opacity: controlled by parent to create depth-fade effect for ancestor lines.
+ * Hover: coordinated via ThreadHoverContext so all segments of the same
+ * ancestor line highlight together across the full thread height.
  */
 
 import { CaretDown, CaretRight } from '@phosphor-icons/react'
+import { useThreadHover } from '@/context/thread-hover-context'
 
 interface ThreadLineProps {
   expanded: boolean
   onToggle: () => void
   authorName: string
   replyCount: number
+  /** URI identifying which ancestor this line belongs to. Used for coordinated hover. */
+  ancestorUri: string
   opacity?: number
   showChevron?: boolean
   /** Width in pixels. Matches the indent step so lines ARE the indentation. */
@@ -23,18 +27,27 @@ export function ThreadLine({
   onToggle,
   authorName,
   replyCount,
+  ancestorUri,
   opacity = 1,
   showChevron = true,
   width = 22,
 }: ThreadLineProps) {
+  const { hoveredUri, setHovered } = useThreadHover()
+  const isHighlighted = hoveredUri === ancestorUri
+
   const label = expanded
     ? `Collapse thread by ${authorName}, ${replyCount} ${replyCount === 1 ? 'reply' : 'replies'}`
     : `Expand thread by ${authorName}, ${replyCount} ${replyCount === 1 ? 'reply' : 'replies'}`
 
+  const chevronColor = isHighlighted ? 'text-accent-foreground' : 'text-muted-foreground'
+  const lineColor = isHighlighted ? 'bg-accent-foreground' : 'bg-border'
+
   return (
     <button
       type="button"
       onClick={onToggle}
+      onMouseEnter={() => setHovered(ancestorUri)}
+      onMouseLeave={() => setHovered(null)}
       aria-expanded={expanded}
       aria-label={label}
       title={expanded ? 'Collapse thread' : 'Expand thread'}
@@ -42,7 +55,9 @@ export function ThreadLine({
       style={{ width }}
     >
       {showChevron && (
-        <span className="absolute left-1/2 top-1 -translate-x-1/2 text-border transition-colors group-hover:text-accent-foreground">
+        <span
+          className={`absolute left-1/2 top-1 -translate-x-1/2 transition-colors ${chevronColor}`}
+        >
           {expanded ? (
             <CaretDown className="h-3 w-3" weight="bold" />
           ) : (
@@ -53,7 +68,7 @@ export function ThreadLine({
       {expanded && (
         <span
           aria-hidden="true"
-          className="absolute inset-y-0 left-1/2 w-0.5 -translate-x-1/2 rounded-full bg-border transition-colors group-hover:bg-accent-foreground"
+          className={`absolute inset-y-0 left-1/2 w-0.5 -translate-x-1/2 rounded-full transition-colors ${lineColor}`}
           style={{ opacity, top: showChevron ? '1.25rem' : 0 }}
         />
       )}
diff --git a/src/context/thread-hover-context.tsx b/src/context/thread-hover-context.tsx
new file mode 100644
index 0000000..0827c9b
--- /dev/null
+++ b/src/context/thread-hover-context.tsx
@@ -0,0 +1,29 @@
+'use client'
+
+import { createContext, useContext, useState, useCallback, useMemo } from 'react'
+
+interface ThreadHoverContextValue {
+  hoveredUri: string | null
+  setHovered: (uri: string | null) => void
+}
+
+const ThreadHoverContext = createContext<ThreadHoverContextValue>({
+  hoveredUri: null,
+  setHovered: () => {},
+})
+
+export function ThreadHoverProvider({ children }: { children: React.ReactNode }) {
+  const [hoveredUri, setHoveredUri] = useState<string | null>(null)
+
+  const setHovered = useCallback((uri: string | null) => {
+    setHoveredUri(uri)
+  }, [])
+
+  const value = useMemo(() => ({ hoveredUri, setHovered }), [hoveredUri, setHovered])
+
+  return <ThreadHoverContext.Provider value={value}>{children}</ThreadHoverContext.Provider>
+}
+
+export function useThreadHover() {
+  return useContext(ThreadHoverContext)
+}