diff --git a/gcc/fortran/resolve.cc b/gcc/fortran/resolve.cc
index ba603b4c4072852c5904870d9b05823fcc74b370..f3f3051eb473026ab1d912a15d0af32b3da5e83f 100644
--- a/gcc/fortran/resolve.cc
+++ b/gcc/fortran/resolve.cc
@@ -13871,6 +13871,16 @@ check_formal:
 	    }
 	}
     }
+
+  /* F2018:15.4.2.2 requires an explicit interface for procedures with the
+     BIND(C) attribute.  */
+  if (sym->attr.is_bind_c && sym->attr.if_source == IFSRC_UNKNOWN)
+    {
+      gfc_error ("Interface of %qs at %L must be explicit",
+		 sym->name, &sym->declared_at);
+      return false;
+    }
+
   return true;
 }
 
diff --git a/gcc/testsuite/gfortran.dg/pr85877.f90 b/gcc/testsuite/gfortran.dg/pr85877.f90
new file mode 100644
index 0000000000000000000000000000000000000000..d8f08cbd21004650413549c1c1a8f83137c545de
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/pr85877.f90
@@ -0,0 +1,25 @@
+! { dg-do compile }
+! PR fortran/85877
+! A procedure with the bind(c) attribute shall have an explicit interface
+! Contributed by G. Steinmetz
+
+function f() bind(c)
+  f = 42.
+end
+
+subroutine p
+  bind(c) f     ! { dg-error "must be explicit" }
+  x = f()
+end
+
+function g() bind(c)
+  g = 42.
+end
+
+subroutine s
+  interface
+     function g() bind(c)
+     end function g
+  end interface
+  x = g()
+end