The veteran slugger belted a two-run homer in his initial at-bat with his new team, sparking a five-run first inning that carried the A's to a 6-1 victory over the Seattle Mariners in the opener of a three-game series.
Geovany Soto added a two-run single during the big opening frame and Jason Hammel turned in his best start in an Oakland uniform to help the A's bounce back from this past weekend's four-game sweep at the hands of the AL West- leading Angels.
Oakland mustered a paltry one run total over its three most recent defeats.
Hammel (2-5), meanwhile, limited the Mariners to three hits -- including a solo homer to Brad Miller -- over eight innings in his sharpest effort since coming to Oakland in a trade with the Chicago Cubs in July.
Seattle's Chris Young (12-7) wasn't nearly as effective, recording only two outs while being rocked for five runs on four hits and two walks.
Dunn, obtained from the Chicago White Sox on Sunday in exchange for minor league pitcher Nolan Sanburn, stepped to the plate with two out and Josh Reddick aboard in the first and launched a towering fly ball that cleared the wall in right, staking the A's to a 2-0 lead and paving the way for an eventual breakout by the Oakland bats.
Young then walked two of the next three hitters around a Jed Lowrie single to load the bases for Soto, who ripped a sharp single to left to bring in two more runs and send Young packing. Lucas Luetge entered for Seattle and promptly served up a single to Eric Sogard that put Oakland up 5-0.
That was more than enough for Hammel, who set down 14 of the first 17 Mariner hitters until Miller led off the sixth with a homer to right-center for Seattle's lone offensive highlight.
The A's got that run back in their half of the sixth when Josh Donaldson plated Reddick with a two-out double off Taijuan Walker.
Hammel was perfect after surrendering Miller's blast, ending his outing by getting nine straight outs.
Dunn finished 2-for-3 while Lowrie went 2-for-4 in his first game since Aug. 13, having missed nearly three weeks with a fractured right index finger ... Seattle's Robinson Cano went 0-for-3 to have a 21-game hitting streak against the A's halted, while teammate Austin Jackson had an 11-game overall tear end after ending 0-for-4 ... Walker, one of several players recalled from Triple-A Tacoma by Seattle before the game, worked six innings and permitted just one run while fanning five ... Donaldson is 13-for-29 with five RBI over an eight- game hit streak.