The Japanese slugger's four-legged friend was the star of the night when L.A. hosted the Baltimore Orioles -- not only did he participate in the pre-game festivities ... but the team handed out 40,000 bobbleheads featuring Ohtani holding Decoy.

Prior to the first inning, Ohtani and Decoy -- wearing a No. 17 jersey -- went out to the field to do the ceremonial toss ... with Shohei setting him up on the mound and placing a baseball next to him before taking his position behind home plate.

Then on Ohtani's command, Decoy picked up the ball and ran toward his papa ... executing a truly memorable first pitch!!

Ohtani was jolly as could be about Decoy's job well done ... giving him a high five and a treat for being the best boy.

The sellout crowd was all about it ... cheering for the adorable pup.

Of course, the Kooikerhondje has become quite the celebrity since he was introduced to the world ... and there was even a bit of mystery surrounding his name before Ohtani finally spilled the beans on it at his introductory press conference with the Dodgers.

His popularity -- as well as Ohtani's -- was evident on Wednesday ... when thousands of fans hit up the stadium hours before the gates opened just to make sure they got their hands on the special giveaway.

Some even got lucky enough to get the golden version of the bobbles ... and those are fetching more than $1,000 on auction sites right now.