Arizona Diamondback Jake Lamb got HYPED this weekend ... watching his little bro advance to the College World Series -- and the awesome scene was caught on video.

Washington rallied to beat Cal State Fullerton in 10 innings Sunday night ... using a sacrifice fly to score the winning run -- and Lamb's reaction was priceless.

FYI -- Lamb -- a 3rd baseman on the D'Backs -- used to play for the Huskies ... and now his younger bro, Dylan, is a freshman reliever for the squad. All of that added up to Jake going ballistic when the winning run crossed home plate.

"My little brother achieved one of my childhood dreams tonight," Lamb wrote of the reaction. "I post this video because I’m at a loss for words. I love you man @dylanrlamb."

Washington's gotta win a few more games to be crowned champs ... but if they do -- we're hoping someone has a camera on Jake for it.