diff --git a/runtime/lua/vim/diagnostic.lua b/runtime/lua/vim/diagnostic.lua
index 39ef18e0b4..10522ab7b1 100644
--- a/runtime/lua/vim/diagnostic.lua
+++ b/runtime/lua/vim/diagnostic.lua
@@ -1824,10 +1824,13 @@ function M.open_float(opts, ...)
     -- Resolve options with user settings from vim.diagnostic.config
     -- Unlike the other decoration functions (e.g. set_virtual_text, set_signs, etc.) `open_float`
     -- does not have a dedicated table for configuration options; instead, the options are mixed in
-    -- with its `opts` table which also includes "keyword" parameters. So we create a dedicated
-    -- options table that inherits missing keys from the global configuration before resolving.
-    local t = global_diagnostic_options.float
-    local float_opts = vim.tbl_extend('keep', opts, type(t) == 'table' and t or {})
+    -- with its `opts` table. We create a dedicated options table (`float_opts`) that inherits
+    -- missing keys from the global configuration (`global_diagnostic_options.float`), which can
+    -- be a table or a function.
+    local o = global_diagnostic_options
+    local t = type(o.float) == 'table' and o.float
+      or (type(o.float) == 'function' and o.float(0, bufnr) or {})
+    local float_opts = vim.tbl_extend('keep', opts, t)
     opts = get_resolved_options({ float = float_opts }, nil, bufnr).float
   end
 
diff --git a/test/functional/lua/diagnostic_spec.lua b/test/functional/lua/diagnostic_spec.lua
index e52a8991ce..765fb70183 100644
--- a/test/functional/lua/diagnostic_spec.lua
+++ b/test/functional/lua/diagnostic_spec.lua
@@ -1750,6 +1750,19 @@ describe('vim.diagnostic', function()
           return _G.count_extmarks(_G.diagnostic_bufnr, _G.diagnostic_ns)
         end)
       )
+
+      --`float` option defined as a function.
+      eq(
+        true,
+        exec_lua(function()
+          vim.diagnostic.config({
+            float = function(_, _)
+              return { border = 'rounded' }
+            end,
+          })
+          return type(vim.diagnostic.config().float) == 'function'
+        end)
+      )
     end)
 
     it('can use functions for config values', function()
@@ -2501,6 +2514,37 @@ describe('vim.diagnostic', function()
       )
     end)
 
+    it('', function()
+      eq(
+        { { 'Diagnostics:', '1. syntax error' }, '╭', '┌' },
+        exec_lua(function()
+          local diagnostics = {
+            _G.make_error('syntax error', 0, 1, 0, 3),
+          }
+          vim.api.nvim_win_set_buf(0, _G.diagnostic_bufnr)
+          vim.diagnostic.set(_G.diagnostic_ns, _G.diagnostic_bufnr, diagnostics)
+          vim.diagnostic.config({
+            float = function(_, bufnr)
+              return { border = bufnr == _G.diagnostic_bufnr and 'rounded' or 'single' }
+            end,
+          })
+          local float_bufnr, winnr = vim.diagnostic.open_float()
+          local lines = vim.api.nvim_buf_get_lines(float_bufnr, 0, -1, false)
+          local border1 = vim.api.nvim_win_get_config(winnr).border
+          vim.api.nvim_win_close(winnr, true)
+
+          local other_bufnr = vim.api.nvim_create_buf(true, true)
+          vim.api.nvim_win_set_buf(0, other_bufnr)
+          vim.diagnostic.set(_G.diagnostic_ns, other_bufnr, diagnostics)
+          local _, winnr1 = vim.diagnostic.open_float()
+          local border2 = vim.api.nvim_win_get_config(winnr1).border
+          vim.api.nvim_win_close(winnr1, true)
+
+          return { lines, border1[1], border2[1] }
+        end)
+      )
+    end)
+
     it('can show diagnostics from the whole buffer', function()
       eq(
         { '1. Syntax error', '2. Some warning' },