Saito, 41, went 4-2 with a 2.03 earned run average in 30 relief appearances with the Milwaukee Brewers last season. He was also 1-0 with seven scoreless innings in six appearances during the postseason.
Over parts of six major league seasons with the Los Angeles Dodgers, Boston Red Sox, Atlanta Braves and Brewers, he is 21-15 with 84 saves and a 2.18 ERA in 322 appearances out of the bullpen. He has also posted a 1.69 ERA in 10 career postseason games.
"Takashi brings a great veteran presence to our bullpen and strengthens an area where we already feel very good about our options," Diamondbacks general manager Kevin Towers said. "He has closed, been a set-up man, and is versatile enough to fill any role."
The right-handed Saito set a Dodgers rookie record with 24 saves in 2006, and converted 47 of his first 50 major league save opportunities.
Saito will become the first player of Japanese descent to play for the Diamondbacks.