James Neal launched a shot from the point that Paul Martin deflected behind the Washington net. The puck bounced off the backboards and onto the stick of Malkin, who was stationed to the right of the goal.
The NHL's leading scorer tucked it into the open net for his 26th tally of the season as the Penguins reeled off a sixth straight victory.
Neal finished with a pair of goals -- including the third-period equalizer -- and an assist while Malkin added two helpers for the Penguins.
Michal Neuvirth turned aside 23-of-27 shots for the Capitals.