Bruce connected off the first pitch from lefty reliever Tim Byrdak leading off the bottom of the ninth inning, sparking a wild celebration at home plate while fireworks lit the night sky over Cincinnati's Great American Ball Park.
"My boys are so happy," Reds manager Dusty Baker told a TV reporter. "The whole city is happy. To win it at home, it's just great."
Bruce was 0-for-3 with two strikeouts and a double play against Astros starter Wandy Rodriguez before stepping to the plate in the ninth against Byrdak.
"It was only fitting that Jay Bruce hit that home run to win this game," said a giddy Baker. "I love this team. I love Cincinnati."
Flame-throwing Aroldis Chapman, the 22-year-old rookie from Cuba, was the winning pitcher after a scoreless ninth inning. Chapman (2-2) struck out his first two batters, topping 100-mph on the radar gun, and induced a groundout to short.
The Reds secured the division crown by virtue of an 88-69 record that put them out of reach of the second place St Louis Cardinals (81-75).
Cincinnati joined NL East champion Philadelphia Phillies to form half of the National League's post-season field.
(Reporting by Larry Fine, Editing by Frank Pingue)
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