On Monday, he announced his signing with UCAM Murcia. He reached a three-year deal, according to EuroHoops.net.
"Big move @alex_ante34 I'm so happy and proud of you," Giannis congratulated him via Twitter. "Now let's get it."
Despite going unranked on the 2020 ESPN 100 list, Alex was twice a first-team all-state selection who averaged 20.3 points and 7.3 rebounds per game during his senior season at Dominican High School in Wisconsin.
"If Alex can stay on the path and continue to respect the process of getting good, he will not be far away," Dominican High coach Jim Gosz said.
"He is a student of the game, and I can see him really being good in a couple years. Once Alex sees his body take off from strength and conditioning work ... lookout!"
The 18-year-old reportedly received college offers from DePaul, Ohio and Green Bay before electing to go pro instead of entering college.
"I don't really believe in pressure like that, but it's definitely a lot of expectations on you. And being the baby brother, it's a lot of being told what to do a lot of the times, but you've just got to take the criticism and move on," Alex told ESPN in December.
Alex will be eligible for the NBA draft in 2021.
"I'll tell you one thing. It's hard to be Alex. It's hard to have three brothers get drafted. He wants to get drafted, but he's got to get better every day," Giannis told ESPN at halftime of a Dec. 10 game at Dominican. "There's going to be down days, there's going to be hard days, it's going to be days that he doesn't have confidence, but that's where we step in and say, 'Hey, you're going to be good, keep working hard. Keep your head up. You're going to be really good.'
"But there's going to be other days where he's playing well like today, and we've got to find things to tell him that he sucks like, 'Your hands are down on defense. You're not rebounding the ball.' We've got to find ways to keep him humble, so that's the only way you can get better."