The club wouldn't confirm specific interest in Ohtani or any other player, but Dipoto was spotted on a televised broadcast of Ohtani's start for the Hokkaido Nippon Ham Fighters against the Tohoku Rakuten Golden Eagles at the Sapporo Dome.
Ohtani hasn't announced whether he'll opt for the Major Leagues next season yet or not, but he is an intriguing international prospect, given his ability to hit 100-plus mph as well as a highly regarded bat that could allow him to be a two-way player in the right situation in the Majors.
Ohtani, 23, has been dealing with an ankle injury much of this season, but worked 5 2/3 innings to pick up his first win in Tuesday's 7-0 victory, allowing one hit and three walks and registering four strikeouts while touching 163 kilometers per hour (101.3 mph) on the radar gun at the Sopporo Dome.
Japanese media reports said scouts and executives from about half of the MLB's 30 teams were present.
The Mariners have a strong history of Japanese players on their Major League roster, due in part to Japanese ownership through Nintendo from 1992-2016. Nintendo of America sold its majority ownership last year, but still has a minority interest and the Mariners certainly have history with Japanese players.
Ichiro Suzuki was a Mariners star from 2001-12 and the club has had at least one Japanese player on its roster every season since 1998, including current pitcher Hisashi Iwakuma, who has been a rotation stalwart since 2012 before running into injury issues this year.
Mac Suzuki was the first Japanese player with the Mariners in 1996 and he also pitched for Seattle in 1998-99. Kazahiro Sasaki was the closer from 2000-03, with Shigetoshi Hasegawa pitching on the 2002-03 clubs.
Kenji Johjima worked as Seattle's catcher from 2006-09, Munenori Kawasaki was an infielder in 2012 and Norichika Aoki played left field last year.
Ohtani, who stands 6-foot-4 and throws right-handed and bats left-handed, has many questions surrounding his situation, given he could make considerably more money by waiting two years until he turns 25 before opening himself to MLB bidding.
There's also the debate of whether he's best suited to be strictly a pitcher, or outfielder, or a pitcher who plays the outfield or designated hitter on his off-days. It all makes for a lot of talk at the moment, as well as a lot of teams keeping a close eye on the proceedings.
• Manager Scott Servais said the Mariners still haven't settled on a Sunday starter yet for the Astros series this weekend. James Paxton will make his return in Friday's series opener in Houston, with Erasmo Ramirez going Saturday. Ariel Miranda would be in line for Sunday's start, but rookie Andrew Moore is under consideration for that spot now after allowing just one hit in six innings in relief of Miranda in Monday's 5-3 loss.
"We haven't finalized anything yet," Servais said. "I want to give it a couple days and see where we're at. We'll see how [Miranda] feels today and tomorrow and talk to the trainers. Nothing has been set up yet. Our rotation has been hit and miss. With Felix and Pax coming back, it gives us some room to move some guys around."