The AL West champion Mariners sent reliever Jose Paniagua and prospects Dennis Stark and Brian Fuentes to Colorado for the two-time All-Star.
There has been speculation the Rockies will now trade Paniagua to Detroit for all-purpose player Shane Halter.
Cirillo hit .312 with a career-high 17 home runs and 83 RBIs last season. He led major league third baseman with a .982 fielding percentage.
"We are very pleased to finalize this deal, cementing Jeff as our third baseman for the next several seasons," Seattle general manager Pat Gillick said. "One of our priorities this offseason was to fill our need at third. Jeff was our top choice."
Cirillo will replace David Bell, who became a free agent after hitting .260 with 15 home runs and 64 RBIs for Seattle.
Colorado and Seattle agreed Thursday on the four players in the trade during the winter meetings in Boston. The Mariners then got permission from the commissioner's office for a 36-hour window to talk to Cirillo, hoping to restructure his contract.
Cirillo is guaranteed $27.6 million over the next four seasons, and there is a club option for a fifth year at $7.5 million with a $1.25 million buyout.
The Mariners were hoping to defer some of the early dollars, but wound up leaving intact the money portion of his deal. Allowed to talk to Cirillo until 3 a.m. PST, they reached agreement - reworking the portion of his contract involving future trades - shortly after midnight.
Cirillo, 32, is a career .311 hitter since making his major league debut with Milwaukee in 1994. A line-drive hitter, the Mariners hope he will not be hurt by leaving offense-oriented Coors Field.
"His bat fits really well in our lineup and his stroke is a nice fit for Safeco," Mariners manager Lou Piniella said.
Cirillo finished this year with 85 straight errorless games, setting an NL record at third base. The major league mark for consecutive games without an error at third base is 99, set by John Wehner from 1992-2000.
The trade sent Cirillo close to home. His off-season residence is in the Seattle suburb of Redmond.
The Mariners won an AL-record 116 games last season and eventually lost in the ALCS to the New York Yankees.
Paniagua, 28, was part of a deep Mariners bullpen. The right-hander was 4-3 with three saves and a 4.36 ERA in 60 games.
Stark, 27, was 14-2 with a 2.37 ERA at Triple-A Tacoma. The right-hander was 1-1 in four games for Seattle.
Fuentes, a 26-year-old lefty, was 3-2 with six saves and a 2.94 ERA at Tacoma.