Giambi hit a three-run drive into the second deck in right field in the first inning off Kyle Kendrick. He lined a two-run shot off Kendrick in the third. Giambi hit another two-run shot off Danys Baez in the fifth.
He struck out in his final two at-bats and the Rockies won 7-1.
It's the first time the 40-year-old former AL MVP has connected three times in one game. He has 419 career homers and 39 multihomer games. The seven RBIs have tied a career high.
Giambi is the 12th Rockies player to go deep three times in a game. Larry Walker was the last to do it on June 25, 2004, at Cleveland.
Giambi, starting at first base in place of Todd Helton, entered the game 3 for 26 this season.
Information from The Associated Press was used in this report.