KIEV, Ukraine—Joe Biden is on his fifth trip to Ukraine as vice president, pressing the pro-Western government to root out widespread corruption, but activists here say that message is being undermined as his son receives money from a former Ukrainian official who is being investigated for graft.

Since May of last year, Hunter Biden, the vice president’s son, has served as an independent director of Ukrainian gas firm Burisma Holdings Ltd., a closely held company run by a former government official whom Ukrainian and British authorities are investigating for alleged criminal wrongdoing.

Mykola Zlochevsky, who served as ecology and natural resources minister under the Ukrainian government that was ousted in last year’s pro-Western uprising, hasn’t been charged and denies wrongdoing.