On December 22, 2011, DeRosa signed a one-year deal with the Washington Nationals.

On April 29, 2012, DeRosa was placed on the 15-day DL with an oblique strain to make room for first-base/outfield prospect Tyler Moore.
He was hitting .081, or 3-37, with 2 RBI in 13 games.
He returned from the disabled list on September 1.

Despite DeRosa's limited playing time, he became an important mentor for his Nationals teammates, many of whom are much younger.
The day after the Nationals lost the game five of the divisional series to the St. Louis Cardinals reporters asked him if he was going to put a uniform on again (to retire) and DeRosa stated "I don't know if I'm okay with that yet."
On December 22, 2011, DeRosa signed a one-year deal with the Washington Nationals.
On April 29, 2012, DeRosa was placed on the 15-day DL with an oblique strain to make room for first-base/outfield prospect Tyler Moore. He was hitting .081, or 3-37, with 2 RBI in 13 games. He returned from the disabled list on September 1.
Despite DeRosa's limited playing time, he became an important mentor for his Nationals teammates, many of whom are much younger. The day after the Nationals lost the game five of the divisional series to the St. Louis Cardinals reporters asked him if he was going to put a uniform on again (to retire) and DeRosa stated "I don't know if I'm okay with that yet."
Mark DeRosa