The Bucks -- entering the game on a two-game losing streak -- rallied from a 23-point deficit to clinch the top seed. Milwaukee opened the second half with a 20-5 to get back into the contest, and then closed the game with another 20-5 run to turn it into a rout.
"It was just effort," Bucks sharpshooter Khris Middleton said. "We all spoke about it [at halftime] and realized we just weren't playing our best basketball. We were one or two steps behind on everything. So our focus was just play harder, play better."
Middleton tied with Bucks superstar Giannis Antetokounmpo to lead the team with 33 points.
The short-handed Heat, playing without Jimmy Butler (foot) and Goran Dragic (ankle), stormed out to a 17-point lead at halftime after hitting 13 3-pointers in the opening half. But the Bucks emerged from the locker room with a renewed effort on defense, swarming whoever had the ball for the Heat.
"You have to give them credit," Heat head coach Erik Spoelstra said. "They stepped up their pressure. We felt that more in the second half. And we didn't help ourselves, either, with the fouling, particularly in the third quarter. We were hacking and reaching.
"That allowed them to slow the game down a little bit and get to the free throw line and just kind of crawl back into it, but you have to give them credit, for sure."
Duncan Robinson led the Heat with 21 points on 5-of-9 shooting from 3-point range. Tyler Herro chipped in 20 points in 33 minutes off the bench.
Bucks big man Brook Lopez had 17 points and four rebounds, while Pat Connaughton added 11 points off the bench.
The Bucks claimed the No. 1 seed in the East with four games to spare. The Los Angeles Lakers already secured the top seed in the Western Conference.
The NBA playoffs are set to begin Aug. 17.