The reality star, 42, shared plenty of photos from their day, quipping in one of her first Instagram Story posts, "More soccer mom content coming up live from Japan."
The twosome was joined by another mother-son duo as they took in the game and then had the opportunity to greet a few of the players, including Ronaldo and Neymar Jr.
At point, an ecstatic Saint and his pal enjoyed some private time with Ronaldo, 38, who did a cute handshake with the famous offspring.
He also signed autographs and posed for photos before heading out.
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A few days earlier, the gang was in the crowd to watch Lionel Messi's first MLS game in Miami, Fla. — a special treat for the tot.
"He's obsessed with soccer. And I will do everything for my babies. So I've traveled the world," Kardashian explained to an Apple TV reporter of trotting around for her eldest son. "We do soccer trips and it's not stopping all summer. We're going to different exhibition games."
At one point during her interview, Saint ran up to the SKIMS founder to show that he had "the players' sweat on him."
"They're so excited. That just makes me so excited," she added of watching the kids' enthusiasm.
Kardashian revealed she's been having a blast as well, especially since she used to play the sport as a kid! "I wasn't bad," the bombshell declared. "I loved it. So I'm happy that my boy loves it and that we can come and hang out."
It looks like the makeup mogul's other three children — daughters North West, 10, and Chicago, 5, as well as son Psalm, 4 — stayed put in California.
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E! News reported on Kardashian's comments about their love for soccer.