The Rockies sent minor league right-hander Bruce Billings and a player to be named to Oakland for the 34-year-old Ellis and cash.
Colorado has searched for stability at second base all season, trying out Jose Lopez -- before he was designated for assignment last month -- along with Chris Nelson and Jonathan Herrera. Eric Young Jr. has also played the position.
"I think the message with this acquisition of a veteran, proven major league player is there's no scholarship program here," Rockies manager Jim Tracy said after a 6-4 loss to the Chicago White Sox in 10 innings Thursday.
"This is not a feel-good situation, where you try really hard and if you're failing or doing this or that, then it's going to be tolerated," he said.
The plan was to insert Ellis in the lineup as soon as possible, maybe even Friday night against the Kansas City Royals.
"Slot him at second in the lineup and play him at second base," Rockies general manager Dan O'Dowd said. "He'll add some toughness to our club both mentally and physically."
A career .265 hitter, Ellis is batting just .217 this season. He went on the disabled list with a hamstring injury earlier this season and when he returned, rookie Jemile Weeks was Oakland's new starter at second base.
"He's obviously struggling offensively, so he'll fit right in," chuckled O'Dowd, making light of the hitting woes of his team. "He's no pixie dust here, we're going to have to continue to grind through some issues we have. But we certainly like the player."
Ellis looked forward to a new challenge.
"I've been proud to have been a part of it for 10 years. It's going to be tough," he said. "There's a time in everybody's life where this stuff happens. This just happens to be my time to move on.
"I've had so many good memories. I had three children here, I had a lot of postseason games and great memories of being in this ballpark and in this organization. At the same time, I still see myself as an every-day second baseman and this is an opportunity to do that."
Billings has spent most of the season at Triple-A Colorado Springs, going 6-2 with a 4.47 ERA. Billings made his major league debut on May 27 against the St. Louis Cardinals and surrendered one run in two innings.
There may be more moves on the horizon as the Rockies try to climb back into the race in the NL West. They remain 6½ games behind the San Francisco Giants.
"We're going to continue to look for pitching and another bat," O'Dowd said. "Trying to take one thing at a time as they get presented to us."
Ellis was the longest-tenured member of the A's following the departure of six-time Gold Glove third baseman Eric Chavez.
Now, all those familiar old faces on the roster from the past decade are gone. So many others departed before Ellis did: Jason Giambi, Miguel Tejada, Johnny Damon and the Big Three pitching sensation of Barry Zito, Tim Hudson and Mark Mulder.
Ellis was reunited with Giambi on the Rockies.
"Obviously, it's kind of mixed emotions on this given what Mark has meant to the franchise over the years, not just on the field but off the field," Oakland general manager Billy Beane said. "He's probably been as good a representative that we've had since I've been here.
"It's with mixed emotions, but at end of the day I think this is a great situation for Mark," he said.