The former All-Star went 4-2 with a 2.03 ERA in 30 relief appearances for the NL Central-champion Milwaukee Brewers last season.
Saito is 21-15 with 84 saves and a 2.18 ERA over parts of six seasons with the Los Angeles Dodgers, Boston, Atlanta and Milwaukee. Opponents have hit .199 against him with 389 strikeouts and 103 walks in 322 games.
Saito's best season was as a Dodgers closer in 2006, when he had 47 saves in 50 opportunities.
The signing was the latest in a series of moves intended to bolster the pitching staff of the reigning NL West champions. The team acquired right-handed starter Trevor Cahill and left-handed reliever Craig Breslow in a five-player deal with Oakland.
Saito brings experience and the ability to help in various situations, Diamondbacks general manager Kevin Towers said.
"Takashi brings a great veteran presence to our bullpen and strengthens an area where we already feel very good about our options," Towers said. "He has closed, been a setup man and is versatile enough to fill any role."
Since 2006, Saito ranks fifth among relievers with a 2.18 ERA. In 10 postseason games, he is 1-0 with a 1.69 ERA.
He spent 14 seasons with Yokohama of the Japanese Central League before coming to the United States. Saito will be the first Japanese player to play for the Diamondbacks.