Clippard, who turns 31 on Sunday, pitched for the A's and Mets last season, going a combined 5-4 with a 2.92 ERA in 69 appearances.
The right-hander has a 2.88 ERA over his nine-year big league career, seven of which were with the Nationals.
Adding an arm to the bullpen is the one piece the D-backs have still been seeking after adding a pair of big-ticket starting pitchers this winter in Zack Greinke and Shelby Miller.
Clippard has 53 saves in his career, including 19 last season, and he gives the D-backs another option for the closer's role should the team decide that Brad Ziegler is more valuable as a setup man.