In a letter sent to the adult film actress -- obtained by TMZ -- PETA invites SD to collaborate on its next PSA, which rails against horse racing.

The campaign is set to release ahead of the Belmont Stakes and references Stormy's past sex confession involving the 45th prez ... where she alleged Trump lasted no more than 2 minutes in bed.

Alongside a mockup of the PSA, a tagline reads ... "Nobody likes a two-minute ride. Turn your back on horse racing cruelty."

PETA applauds Stormy for speaking out, and encourages her to use her platform for a good cause, noting ... "Every week, an average of 24 horses shatter bones and die at racetracks across the country, and the distressing number of deaths continue to mount. Last year alone, 16 horses died at Churchill Downs in the weeks leading to the Kentucky Derby."

The org says Stormy could make a significant difference if she chooses to participate in the campaign.

She is a key witness in Trump's hush money trial ... where she testified about the $130,000 NDA she signed with DT's then-attorney Michael Cohen in 2016. Remember, Trump is standing trial for allegedly falsifying business records in an effort to hide the hush money payment made to Stormy.

Stormy later took a jab at Trump for not taking the stand in his trial, writing on X ... "Real men respond to testimony by being sworn in and taking the stand in court. Oh...wait. Nevermind."