diff --git a/runtime/lua/vim/lsp/util.lua b/runtime/lua/vim/lsp/util.lua
index 52a6fe89f32a79..5a68138f1e2ebc 100644
--- a/runtime/lua/vim/lsp/util.lua
+++ b/runtime/lua/vim/lsp/util.lua
@@ -735,6 +735,7 @@ function M.fancy_floating_markdown(contents, opts)
   local bufnr = api.nvim_create_buf(false, true)
   local winnr = api.nvim_open_win(bufnr, false, M.make_floating_popup_options(width, height, opts))
   vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, stripped)
+  api.nvim_buf_set_option(bufnr, 'modifiable', false)
 
   -- Switch to the floating window to apply the syntax highlighting.
   -- This is because the syntax command doesn't accept a target.