Slattery, 52, is joining the upcoming comedy film in an undisclosed role, according to Entertainment Weekly.
He will be appearing alongside the recently-cast Morgan Freeman, Dennis Haysbert and Liam Neeson in the sequel to 2012's 'Ted'.
Mark Wahlberg is back as the hapless John Bennett, while MacFarlane will provide the voice of the talking plush bear Ted.
Amanda Seyfried will appear as the new female lead, stepping in for the first movie's star Mila Kunis.
Seyfried worked with MacFarlane on his most recent film 'A Million Ways to Die in the West'.