Littell agreed to a contract with the Diamondbacks on Sunday, Nick Piecoro of the Arizona Republic reports.
The veteran right-hander was let go by the Nationals on Friday after posting a 4.97 ERA, 1.34 WHIP and 70:35 K:BB across 112.1 innings this year. Arizona's starting rotation is plenty banged up, though Michael Soroka (glute) is poised to come off the injured list Sunday. In addition to the uncertainty of his role, it's not clear whether Littell inked a major-league or minor-league contract.