The 34-year-old will compete for a spot in the back end of the Padres' bullpen. He was 2-5 with a 4.95 ERA with the Washington Nationals last season.
He had a career-high 34 saves for Toronto in 2013.
Janssen said he was throwing a bullpen at a high school near where he lives in Manhattan Beach, California, on Thursday when he got a call from his agent that he was talking with the Padres. A deal came together quickly.
Says Janssen, "I've pitched everywhere in a bullpen, from early innings to the back end to closing. I truly believe that if you can pitch in the sixth, you can pitch in the ninth, and vice versa."