diff --git a/doc/src/sgml/libpq.sgml b/doc/src/sgml/libpq.sgml
index 2f9350b10e1..3a470fefac1 100644
--- a/doc/src/sgml/libpq.sgml
+++ b/doc/src/sgml/libpq.sgml
@@ -6944,6 +6944,9 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
       </indexterm>
       <envar>PGREQUIRESSL</envar> behaves the same as the <xref
       linkend="libpq-connect-requiressl"> connection parameter.
+      This environment variable is deprecated in favor of the
+      <envar>PGSSLMODE</envar> variable; setting both variables suppresses the
+      effect of this one.
      </para>
     </listitem>
 
diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index 4f13624017a..8c7169e171d 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -4635,6 +4635,30 @@ conninfo_add_defaults(PQconninfoOption *options, PQExpBuffer errorMessage)
 			}
 		}
 
+		/*
+		 * Interpret the deprecated PGREQUIRESSL environment variable.  Per
+		 * tradition, translate values starting with "1" to sslmode=require,
+		 * and ignore other values.  Given both PGREQUIRESSL=1 and PGSSLMODE,
+		 * PGSSLMODE takes precedence; the opposite was true before v9.3.
+		 */
+		if (strcmp(option->keyword, "sslmode") == 0)
+		{
+			const char *requiresslenv = getenv("PGREQUIRESSL");
+
+			if (requiresslenv != NULL && requiresslenv[0] == '1')
+			{
+				option->val = strdup("require");
+				if (!option->val)
+				{
+					if (errorMessage)
+						printfPQExpBuffer(errorMessage,
+										  libpq_gettext("out of memory\n"));
+					return false;
+				}
+				continue;
+			}
+		}
+
 		/*
 		 * No environment variable specified or the variable isn't set - try
 		 * compiled-in default