Ambrose took the lead from Busch with 19 laps to go.
"I knew I had to dive-bomb him to try and get past," Ambrose said. "If I would have waited for the last 10 laps, he was going to do anything he could to block me, so I just tried to surprise him."
It marked Ambrose's his second career Nationwide victory.
He had won a NASCAR national touring series race for the first time a year ago at Watkins Glen.
Busch fell to third after Ambrose passed him but fought off Carl Edwards in the closing laps to finish second. Busch has now finished first or second for the 10th straight race.
Edwards finished third, with pole sitter Kevin Harvick fourth and Ron Fellows fifth.
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