diff --git a/CHANGELOG.md b/CHANGELOG.md
index dd24c4ef6..60c477454 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,7 @@
 
 #### Fixes
 
+* [#2588](https://github.com/ruby-grape/grape/pull/2588): Fix defaut format regression on */* - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 ### 2.4.0 (2025-06-18)
diff --git a/lib/grape/middleware/formatter.rb b/lib/grape/middleware/formatter.rb
index f69ac3563..65ffe5378 100644
--- a/lib/grape/middleware/formatter.rb
+++ b/lib/grape/middleware/formatter.rb
@@ -7,6 +7,8 @@ class Formatter < Base
         default_format: :txt
       }.freeze
 
+      ALL_MEDIA_TYPES = '*/*'
+
       def before
         negotiate_content_type
         read_body_input
@@ -138,7 +140,7 @@ def format_from_extension
 
       def format_from_header
         accept_header = env['HTTP_ACCEPT'].try(:scrub)
-        return if accept_header.blank?
+        return if accept_header.blank? || accept_header == ALL_MEDIA_TYPES
 
         media_type = Rack::Utils.best_q_match(accept_header, mime_types.keys)
         mime_types[media_type] if media_type
diff --git a/spec/grape/middleware/formatter_spec.rb b/spec/grape/middleware/formatter_spec.rb
index cb353aa4e..dda260449 100644
--- a/spec/grape/middleware/formatter_spec.rb
+++ b/spec/grape/middleware/formatter_spec.rb
@@ -22,10 +22,14 @@
     context 'default format' do
       let(:body) { ['foo'] }
       let(:env) do
-        { Rack::PATH_INFO => '/somewhere', 'HTTP_ACCEPT' => 'application/json' }
+        { Rack::PATH_INFO => '/somewhere', 'HTTP_ACCEPT' => '*/*' }
       end
 
-      it 'calls #to_json since default format is json' do
+      before do
+        subject.options[:default_format] = :json
+      end
+
+      it 'returns JSON' do
         body.instance_eval do
           def to_json(*_args)
             '"bar"'