The contract is worth $2.5 million and just awaits a physical to be finalized.
Lannan will join up with the likes of Roy Halladay, Cole Hamels and Cliff Lee. Lannan was likely brought in to replace Vance Worley, who was dealt to the Minnesota Twins in the offseason to bring in center fielder Ben Revere.
The southpaw had spent his entire major league career with the Washington Nationals up to this point, but didn't get to pitch much last year with the club after being sent to Triple-A.
In his career, the 28 year old is 42-52 with a 4.01 ERA. He's also known to keep the ball on the ground when he has good stuff, something very valuable when your home stadium is Citizens Bank Park.