The Capitals gave up a fourth-round pick in this year's draft for what is essentially a 20-game rental. Penner signed a one-year, $2 million contract last year and is set to become a free agent this summer.
The Capitals began the day in ninth place in the Eastern Conference, in danger of missing the playoffs for the first time since 2007. They have struggled in particular on defence, an area they could address before Wednesday's NHL trade deadline.
Penner, 31, has played for the Los Angeles Kings, Edmonton Oilers and Anaheim over nine NHL seasons. He had 13 goals and 19 assists with the Ducks this season.
He would appear to be a natural fit at left wing on the Capitals' top line. Coach Adam Oates' best options to partner with Ovechkin and Backstrom have been Marcus Johansson, Brooks Laich and Martin Erat, but none of the three has looked suited for the role.
"I look forward to coming in," Penner said, "and hopefully adding something they were looking for."
The deal returns to the Ducks the fourth-round pick they sent to Washington in the trade for Mathieu Perreault in September.