diff --git a/python/py-gmpy2/Portfile b/python/py-gmpy2/Portfile
index b5a9ed845180..8a1a7213bdee 100644
--- a/python/py-gmpy2/Portfile
+++ b/python/py-gmpy2/Portfile
@@ -26,6 +26,9 @@ python.versions     38 39 310 311 312
 if {${name} ne ${subport}} {
     depends_lib-append  port:gmp port:libmpc port:mpfr
 
+    # gmpy2_convert_gmp.c: error: ‘for’ loop initial declaration used outside C99 mode
+    patchfiles-append   patch-c99.diff
+
     post-destroot {
         xinstall -m 0644 -W ${worksrcpath} README.rst \
             ${destroot}${prefix}/share/doc/${subport}
diff --git a/python/py-gmpy2/files/patch-c99.diff b/python/py-gmpy2/files/patch-c99.diff
new file mode 100644
index 000000000000..ef13b9e00b0b
--- /dev/null
+++ b/python/py-gmpy2/files/patch-c99.diff
@@ -0,0 +1,10 @@
+--- setup.py	2024-03-24 02:01:15.000000000 +0800
++++ setup.py	2024-07-16 19:44:11.000000000 +0800
+@@ -6,6 +6,7 @@
+ 
+ ON_WINDOWS = platform.system() == 'Windows'
+ _comp_args = ["DSHARED=1"]
++_comp_args.append('std=c99')
+ sources = ['src/gmpy2.c']
+ if os.getenv('CIBUILDWHEEL'):
+     include_dirs = [os.path.join(os.path.dirname(__file__), '.local', 'include')]