diff --git a/runtime/doc/diagnostic.txt b/runtime/doc/diagnostic.txt
index fe822aeaf6..4a4b2abc92 100644
--- a/runtime/doc/diagnostic.txt
+++ b/runtime/doc/diagnostic.txt
@@ -546,7 +546,8 @@ Lua module: vim.diagnostic                                    *diagnostic-api*
     Fields: ~
       • {bufnr}?          (`integer`, default: current buffer) Buffer number
                           to show diagnostics from.
-      • {namespace}?      (`integer`) Limit diagnostics to the given namespace
+      • {namespace}?      (`integer|integer[]`) Limit diagnostics to the given
+                          namespace(s).
       • {scope}?          (`'line'|'buffer'|'cursor'|'c'|'l'|'b'`, default:
                           `line`) Show diagnostics from the whole buffer
                           (`buffer"`, the current cursor line (`line`), or the
diff --git a/runtime/lua/vim/diagnostic.lua b/runtime/lua/vim/diagnostic.lua
index 5a78ad0d0a..7e0c4dd31a 100644
--- a/runtime/lua/vim/diagnostic.lua
+++ b/runtime/lua/vim/diagnostic.lua
@@ -115,8 +115,8 @@ end
 --- (default: current buffer)
 --- @field bufnr? integer
 ---
---- Limit diagnostics to the given namespace
---- @field namespace? integer
+--- Limit diagnostics to the given namespace(s).
+--- @field namespace? integer|integer[]
 ---
 --- Show diagnostics from the whole buffer (`buffer"`, the current cursor line
 --- (`line`), or the current cursor position (`cursor`). Shorthand versions
diff --git a/test/functional/lua/diagnostic_spec.lua b/test/functional/lua/diagnostic_spec.lua
index 41e85b82f6..ad08a33fc0 100644
--- a/test/functional/lua/diagnostic_spec.lua
+++ b/test/functional/lua/diagnostic_spec.lua
@@ -2997,7 +2997,7 @@ describe('vim.diagnostic', function()
 
     it('allows filtering by namespace', function()
       eq(
-        2,
+        { 'Diagnostics:', '1. Syntax error' },
         exec_lua(function()
           local ns_1_diagnostics = {
             _G.make_error('Syntax error', 0, 1, 0, 3),
@@ -3012,7 +3012,31 @@ describe('vim.diagnostic', function()
             vim.diagnostic.open_float(_G.diagnostic_bufnr, { namespace = _G.diagnostic_ns })
           local lines = vim.api.nvim_buf_get_lines(float_bufnr, 0, -1, false)
           vim.api.nvim_win_close(winnr, true)
-          return #lines
+          return lines
+        end)
+      )
+    end)
+
+    it('allows filtering by multiple namespaces', function()
+      eq(
+        { 'Diagnostics:', '1. Syntax error', '2. Some warning' },
+        exec_lua(function()
+          local ns_1_diagnostics = {
+            _G.make_error('Syntax error', 0, 1, 0, 3),
+          }
+          local ns_2_diagnostics = {
+            _G.make_warning('Some warning', 0, 1, 0, 3),
+          }
+          vim.api.nvim_win_set_buf(0, _G.diagnostic_bufnr)
+          vim.diagnostic.set(_G.diagnostic_ns, _G.diagnostic_bufnr, ns_1_diagnostics)
+          vim.diagnostic.set(_G.other_ns, _G.diagnostic_bufnr, ns_2_diagnostics)
+          local float_bufnr, winnr = vim.diagnostic.open_float(
+            _G.diagnostic_bufnr,
+            { namespace = { _G.diagnostic_ns, _G.other_ns } }
+          )
+          local lines = vim.api.nvim_buf_get_lines(float_bufnr, 0, -1, false)
+          vim.api.nvim_win_close(winnr, true)
+          return lines
         end)
       )
     end)