diff --git a/crypto/x509v3/v3_purp.c b/crypto/x509v3/v3_purp.c
index fff099474e..0820a2a5d3 100644
--- a/crypto/x509v3/v3_purp.c
+++ b/crypto/x509v3/v3_purp.c
@@ -528,6 +528,11 @@ static int check_ca(const X509 *x)
     }
 }
 
+void X509_set_proxy_flag(X509 *x)
+{
+    x->ex_flags |= EXFLAG_PROXY;
+}
+
 int X509_check_ca(X509 *x)
 {
     if (!(x->ex_flags & EXFLAG_SET)) {
diff --git a/doc/crypto/X509_get_extension_flags.pod b/doc/crypto/X509_get_extension_flags.pod
index 2509b65ca0..473ef28b6d 100644
--- a/doc/crypto/X509_get_extension_flags.pod
+++ b/doc/crypto/X509_get_extension_flags.pod
@@ -4,8 +4,8 @@
 
 X509_get0_subject_key_id,
 X509_get_pathlen,
-X509_get_extension_flags, X509_get_key_usage, X509_get_extended_key_usage -
-retrieve certificate extension data
+X509_get_extension_flags, X509_get_key_usage, X509_get_extended_key_usage,
+X509_set_proxy_flag - retrieve certificate extension data
 
 =head1 SYNOPSIS
 
@@ -16,6 +16,7 @@ retrieve certificate extension data
    uint32_t X509_get_key_usage(X509 *x);
    uint32_t X509_get_extended_key_usage(X509 *x);
    const ASN1_OCTET_STRING *X509_get0_subject_key_id(X509 *x);
+   void X509_set_proxy_flag(X509 *x);
 
 =head1 DESCRIPTION
 
@@ -102,6 +103,10 @@ X509_get_extended_key_usage() return an internal pointer to the subject key
 identifier of B<x> as an B<ASN1_OCTET_STRING> or B<NULL> if the extension
 is not present or cannot be parsed.
 
+X509_set_proxy_flag() marks the certificate with the B<EXFLAG_PROXY> flag.
+This is for the users who need to mark non-RFC3820 proxy certificates as
+such, as OpenSSL only detects RFC3820 compliant ones.
+
 =head1 NOTES
 
 The value of the flags correspond to extension values which are cached
@@ -139,7 +144,7 @@ L<X509_check_purpose(3)>
 
 =head1 HISTORY
 
-X509_get_pathlen() was added in OpenSSL 1.1.0.
+X509_get_pathlen() and X509_set_proxy_flag() were added in OpenSSL 1.1.0.
 
 =head1 COPYRIGHT
 
diff --git a/include/openssl/x509v3.h b/include/openssl/x509v3.h
index 89be5f9c14..b37f52bcab 100644
--- a/include/openssl/x509v3.h
+++ b/include/openssl/x509v3.h
@@ -649,6 +649,7 @@ int X509_supported_extension(X509_EXTENSION *ex);
 int X509_PURPOSE_set(int *p, int purpose);
 int X509_check_issued(X509 *issuer, X509 *subject);
 int X509_check_akid(X509 *issuer, AUTHORITY_KEYID *akid);
+void X509_set_proxy_flag(X509 *x);
 
 uint32_t X509_get_extension_flags(X509 *x);
 uint32_t X509_get_key_usage(X509 *x);