diff --git a/runtime/lua/vim/treesitter/highlighter.lua b/runtime/lua/vim/treesitter/highlighter.lua
index 097ad0ea9a..59ee4158f2 100644
--- a/runtime/lua/vim/treesitter/highlighter.lua
+++ b/runtime/lua/vim/treesitter/highlighter.lua
@@ -90,12 +90,26 @@ TSHighlighter.__index = TSHighlighter
 ---           - queries table overwrite queries used by the highlighter
 ---@return vim.treesitter.highlighter Created highlighter object
 function TSHighlighter.new(tree, opts)
-  local self = setmetatable({}, TSHighlighter)
-
-  if type(tree:source()) ~= 'number' then
+  local source = tree:source()
+  if type(source) ~= 'number' then
     error('TSHighlighter can not be used with a string parser source.')
   end
 
+  -- Calling start() again on an already-highlighted buffer must be a no-op: a second instance
+  -- would register duplicate on_bytes/on_changedtree/on_detach callbacks that destroy() never
+  -- unregisters, leaking callbacks that keep firing for the orphaned instance.
+  local existing = TSHighlighter.active[source]
+  if existing then
+    if existing.tree == tree then
+      return existing
+    end
+    -- Different tree (e.g. a language switch): the old instance would otherwise be silently
+    -- orphaned with the same leaked callbacks, so destroy() it first, matching stop() semantics.
+    existing:destroy()
+  end
+
+  local self = setmetatable({}, TSHighlighter)
+
   opts = opts or {} ---@type { queries: table<string,string> }
   self.tree = tree
   tree:register_cbs({
@@ -136,9 +150,6 @@ function TSHighlighter.new(tree, opts)
     end,
   }, true)
 
-  local source = tree:source()
-  assert(type(source) == 'number')
-
   self.bufnr = source
   self.redraw_count = 0
   self._conceal_checked = {}
diff --git a/test/functional/treesitter/highlight_spec.lua b/test/functional/treesitter/highlight_spec.lua
index 87b7c272be..0aa8b9e4df 100644
--- a/test/functional/treesitter/highlight_spec.lua
+++ b/test/functional/treesitter/highlight_spec.lua
@@ -181,6 +181,19 @@ describe('treesitter highlighting (C)', function()
     -- treesitter highlighting is used
     screen:expect(hl_grid_ts_c)
 
+    -- A second start() on an already-highlighted buffer is a no-op: it returns the existing
+    -- highlighter instead of replacing it (replacing it would leak the old instance's
+    -- on_bytes/on_changedtree callbacks, since destroy() is never called on it).
+    eq(
+      true,
+      exec_lua(function()
+        local buf = vim.api.nvim_get_current_buf()
+        local first = vim.treesitter.highlighter.active[buf]
+        vim.treesitter.start()
+        return first == vim.treesitter.highlighter.active[buf]
+      end)
+    )
+
     exec_lua(function()
       vim.treesitter.stop()
     end)