After several more hours of negotiations Sunday, the Red Sox completed a deal for the Padres first baseman.
The Padres and Red Sox had previously reached an agreement in principle on a trade to send the All-Star to Boston for four players, including three minor league prospects.
The Red Sox had received permission to negotiate a contract extension with Gonzalez. But Major League Baseball had set a 2 p.m. ET Sunday deadline to complete the deal, and the trade appeared to be jeopardy when that deadline passed without a formal extension being reached.
The Red Sox have scheduled a Monday news conference to announce the trade.
No contract extension is expected to be announced at the news conference, but both sides have apparently agreed to the parameters of a new deal that is about $161 million for eight years, a seven-year extension at about $154 million, plus the $6.2 million he stood to make in 2011. They can't finalize it yet because of tax considerations for the Red Sox.
The Padres recently exercised Gonzalez's $6.2 million option for 2011, the final year of a team-friendly $15 million, five-year contract he signed in 2007.
The Red Sox and Padres have been discussing Gonzalez for more than a year. The Padres' front office is very familiar with Boston's prospects, as GM Jed Hoyer, assistant GM Jason McLeod and new top executive Josh Byrnes all once worked for Boston.
Gonzalez, the No. 1 overall pick in the 2000 draft, has spent the past five years in San Diego, his hometown, and in the past four seasons has had at least 30 home runs and 99 RBIs.
In five seasons with San Diego, Gonzalez has 161 homers -- two shy of Nate Colbert's club record -- and 501 RBIs. Including parts of two seasons with Texas, he has 168 homers and 525 RBIs.
He set a Padres record with his fourth 30-homer season. His third 100-RBI season tied Phil Nevin for the most in club history.
Gonzalez had surgery to clean up the labrum in his non-throwing right shoulder on Oct. 20. The Padres said then that the three-time All-Star and two-time Gold Glove first baseman was expected to be ready for spring training.
The popular Gonzalez grew up in the San Diego area and was the No. 1 overall pick by Florida in the 2000 amateur draft. He was obtained in a trade with Texas before the 2006 season and has blossomed into one of the game's best first basemen.
Boston was interested in Gonzalez last winter.
While many had expected Gonzalez to be traded before the deadline in July, the Padres were surprise contenders in the NL West and had to keep the slugger.