Saito, 41, was 4-2 with a 2.03 ERA for the Brewers this past season while being limited to 30 games with oblique and calf issues.
"Takashi brings a great veteran presence to our bullpen and strengthens an area where we already feel very good about our options," D-backs general manager Kevin Towers said in a statement.
"He has closed, been a setup man and is versatile enough to fill any role."
The deal is worth a reported $1.75 million.
In six seasons, the Japanese-born righty has a 2.18 ERA and 84 saves. Saito had a career-best 39 saves with the Dodgers in 2007.
The D-backs had been in the market for a veteran reliever to pitch in a setup role, and Saito fits the bill.
The team expressed interest in Saito during last week's Winter Meetings in Dallas, though Towers wanted to wait until he acquired a starting pitcher so he knew how much money he would have left in his budget.
Last week, the D-backs traded for right-handed starter Trevor Cahill and reliever Craig Breslow from the A's in exchange for right-hander pitchers Jarrod Parker and Ryan Cook and outfielder Collin Cowgill. Breslow figures to be the second left-handed reliever in the bullpen, joining Joe Paterson.
As for Saito, who was an All-Star with the Dodgers in 2007, the D-backs get a well-traveled reliever who has played for the Dodgers, Red Sox, Braves and Brewers. He has also fared well in the postseason, going 1-0 with a 1.69 ERA over 10 2/3 innings.
Saito ranks fifth among all Major League relievers since 2006 with a 2.18 ERA, trailing Mike Adams (1.80), Mariano Rivera (1.98), Billy Wagner (2.11) and Joe Nathan (2.16).
Saito, a native of Japan, played 14 seasons for the Yokohama BayStars of the Japanese Central League and was a four-time All-Star (1994, '96, '99 and 2001).
Saito's 2.18 career ERA is the best among all Japanese Major League players, while his 84 career saves rank second to Kazuhiro Sasaki (129). The only Japan-born pitchers to appear in more games than Saito (322) are Shigetoshi Hasegawa (517) and Hideo Nomo (323).