Wes Welker -- BACK IN THE RACE ... 'Undrafted' Returns to Churchill Downs

Wes Welker's horse "Undrafted" is returning to Churchill Downs -- seeking REVENGE after losing a Kentucky Derby pre-race last year ... TMZ Sports has learned.

Welker was the life of the party at Churchill in 2014 -- winning STACKS OF CASH (and then giving cash away) ... while partying his face off with the likes of Tom Brady.

Now, we've learned the NFL star's 5-year-old gelding is registering to hit the track in Kentucky on May 1st -- in the same race he ran last year ... only this time, he wants that victory wreath!!!

No word if Wes plans to break out the khaki suit and watch the race in person -- but he probably should ... 'cause it kinda worked out GREAT last year.