Within the first two innings, the rookie belted two home runs, including his second career grand slam, and drove in six runs. The effort was Bryant's second career multihomer game, and he now leads all Major League rookies as well as all Cubs players with 49 RBIs.
"It was him -- that was our offense tonight," Cubs manager Joe Maddon said. "Without that, this game is entirely different. It was nice to get him going."
Marlins starter Jarred Cosart served up both of Bryant's homers. The Cubs rookie belted a two-run drive in the first and his slam in the second, then made a brief curtain call before the crowd of 37,898.
"Those are always fun," Bryant said of the salute to the fans. "It means you're doing something good."
However, Bryant's second homer was the last Cubs hit of the game. Bryant was called out on strikes while facing David Phelps in the bottom of the fifth, and he flied out to right field against Brad Hand to lead off the eighth.
Bryant's first-inning homer was a mammoth clout to left-center field that was projected by Statcast™ to land 423 feet from the plate.
This slam had to feel more legitimate than Bryant's first, which came on June 17 off Indians outfielder-turned-pitcher David Murphy.
"He definitely has grinded it out and gone through some tough stretches and at-bats," Maddon said of Bryant. "He's made a lot of adjustments at the Major League level, and that's not easy to do. I'm really impressed. Defensively, offensively, the way he's being pitched -- for a first-year guy to be pitched the way he is speaks a lot about what the other team thinks about him.
"He's going to get, like, Spring Training hot. He'll do that again at some point this season. It's just an experience thing. [Rookie] Addison [Russell], too. They're gaining experience. They're handling the day properly, and it's really impressive to watch."
Bryant led the Major Leagues in home runs during Spring Training, belting nine in 40 at-bats over 14 games. He also hit two more in exhibition games that didn't count.
"I feel that way every time I step up to the plate," Bryant said of his spring dominance. "That's a good feeling to have. Obviously, doing that in Spring Training gives me the confidence I can do it here. I felt really good today and am looking forward to continuing that tomorrow."
The Marlins have approached Bryant differently in the two games at Wrigley than they did in Miami in June.
"You have to notice that and make the adjustments," Bryant said.
"You can't get behind in the count," Cosart said. "You saw what Phelps did. He had success against him later. [Bryant] is a great hitter. You just can't get behind.
"He has a lot of home runs this year, a lot of hits this year," Cosart said. "When he's ahead in the count, that's what good hitters do; they know what's coming. He makes you pay for mistakes."
• Bryant is the first Cubs rookie to hit two grand slams in a season since Hall of Famer Billy Williams did so in 1961. The only other Cubs rookie with two in one year was Howard Earl in 1890.
• Bryant joins Anthony Rizzo as the only Cubs player this season with two multihomer games. He's one of three Cubs players since 1914 with two multihomer games in their first 70 career games, joining Jorge Soler (2014) and Mandy Brooks (1925).