diff --git a/lib/mruby/build.rb b/lib/mruby/build.rb
index effeccf3a..995611a4a 100644
--- a/lib/mruby/build.rb
+++ b/lib/mruby/build.rb
@@ -185,10 +185,11 @@ module MRuby
     end
 
     # Set target port names for this build.
-    # Gems with matching ports/<name>/ directories will compile
-    # those platform-specific sources automatically.
+    # Each gem compiles the first matching ports/<name>/ directory;
+    # later names in the list act as fallbacks for gems that don't
+    # ship a port for the earlier names.
     #   conf.ports :esp32
-    #   conf.ports :rp2040, :posix
+    #   conf.ports :rp2040, :posix    # use rp2040 if available, else posix
     def ports(*names)
       @port_names = names.map { |n| n.to_s }
     end
diff --git a/lib/mruby/gem.rb b/lib/mruby/gem.rb
index c0fe806b4..4ad15da60 100644
--- a/lib/mruby/gem.rb
+++ b/lib/mruby/gem.rb
@@ -60,10 +60,16 @@ module MRuby
         @rbfiles = Dir.glob("#{@dir}/mrblib/**/*.rb").sort
         @objs = srcs_to_objs("src")
 
-        # Add platform-specific sources from ports/<name>/ directories
+        # Add platform-specific sources from the first matching
+        # ports/<name>/ directory.  effective_ports is a fallback
+        # chain: later names act as defaults for gems that don't ship
+        # a port for the earlier names.
         build.effective_ports.each do |port|
           port_dir = "#{@dir}/ports/#{port}"
-          @objs += srcs_to_objs("ports/#{port}") if File.directory?(port_dir)
+          if File.directory?(port_dir)
+            @objs += srcs_to_objs("ports/#{port}")
+            break
+          end
         end
 
         @test_preload = nil # 'test/assert.rb'