Feinstein, who is up for re-election in 2012, will chair Boxer's campaign.
A release announcing the leadership role plays up the collaboration between the two Democrats.
"Senator Boxer and I make a great team for California," Feinstein said in a statement. "That's why I'm so proud to chair Senator Boxer's campaign. Barbara is a proven and effective leader for California, and our strong partnership is essential in creating jobs and turning our economy around."
"We've built a strong working partnership, and I want to make sure we continue working side by side as an effective team for California," Boxer added.
Boxer is gearing up for a tough re-election fight, with polls showing all three GOP Senate hopefuls in a close race with the three-term senator.