On Monday night, his autographed Topps rookie card sold for $6,725.
"It's a truly foreign market that is buying his memorabilia," said Rick Probstein, whose company sold the card on eBay. "Aaron Judge and some of the other baseball stars we have seen on the rise have been boosted by a U.S. market alone."
A year after Judge was compared to Babe Ruth due to his larger-than-life stature and frequency of home runs, Ohtani is also compared to the "Bambino," not only hitting blasts but striking out batters.
"The fact that he is excelling at both hitting and pitching has created somewhat of a frenzy," Probstein said.
The red-ink autographed card, from the 2018 Topps Heritage set, could have been bought in the $1,500-to-$2,000 range toward the end of spring training when fans were questioning if the hype was real over Ohtani, whom the Angels acquired by paying a $20 million posting fee to the Nippon-Ham Fighters, his Japanese team.
But Ohtani, 23, has more than matched expectations early in the regular season, going 2-0 with a 2.08 ERA and taking a perfect game into the seventh inning of his most recent start. He has seven hits at the plate, including home runs in three consecutive games.
"His autographed cards have tripled or quadrupled in the last two to three weeks," Probstein said.
Topps, which signed Ohtani to an exclusive autograph and memorabilia deal in January, said Tuesday it has sold 102,501 on-demand cards of Ohtani on its website in the past 32 hours.
Topps priced one autographed card at $4,000. It sold immediately.
Phillies reliever Pat Neshek, a noted card collector, pulled a Topps Heritage signed Ohtani card, one out of 69, out of a pack last week. He sold the card on eBay for $6,101 on Sunday.
With Ohtani homering in three straight games before striking out 12 in seven shutout innings on Sunday, Neshek realizes he may have sold the card too soon.
"Yeah," Neshek told MLB.com with a smile when asked if he regretted already selling the card. "I mean, back in the day I sold an Aaron Rodgers card about his second year in [the NFL]. I think I got like $40 and they were going for like, two or three grand a couple years ago. That was a dumb one.
"That's kind of the name of the game. You see it with the Aaron Judge stuff. His stuff wasn't selling, then when he blew up last year, it kind of revitalized the whole baseball card industry. They had their best year in 20 years last year. It's nice to see people doing that."
The fact Neshek is active in the market is a good sign, according to Clay Luraschi, vice president of product development for Topps.
"When Major League Baseball players are flipping a card for a profit, you know it's big," said Luraschi, who has been with Topps for 18 years.
"We've seen things that look like this -- Ichiro and [Albert] Pujols in 2001 and obviously what we saw with Aaron Judge last year," Luraschi said. "But nothing I've seen in the past has matched what we've seen the last couple weeks with Ohtani."