The club has not confirmed the signing, but Hernandez is expected to arrive at Space Coast Stadium in the next day or two.
The Nats have been looking for a second veteran starter since they signed right-hander Jason Marquis to a two-year deal in December.
Hernandez is coming off a season in which he went a combined 9-12 with a 5.44 ERA for the Mets and Nationals. Washington signed Hernandez as a free agent on Aug. 26, six days after New York released him.
Hernandez reiterated in January that he would like to re-sign with the Nationals, because he believes they have improved their roster this offseason. He pointed to the acquisition of catcher Ivan Rodriguez as the reason he wanted to return to the nation's capital.
Hernandez would compete with at least 10 pitchers for the final two spots in Washington's rotation. Marquis and left-hander John Lannan are locks, while right-hander Craig Stammen is the leading candidate for the third spot.
This is Hernandez's third stint with the club. With 26 victories, Hernandez remains the winningest pitcher in the five-year history of the Nationals, and he remains the leader in innings pitched (441 2/3). Meanwhile, Hernandez's 67 starts and 263 strikeouts rank second on Washington's all-time list.
Hernandez owns the distinction of throwing both the first pitch in Nats history on April 4, 2005, and the first pitch in Washington, D.C., after 33 summers without big league baseball. In 2005, he was the first member of the Nationals to participate in an All-Star Game.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.