Steve-O just built an Olympic-length pool in his Tennessee backyard ... and naturally, he cemented a "Jackass" butt plug into it.

The 82-foot heated lap pool was built by Pool and Spa Depot, and Steve-O tells TMZ it's part of his latest health and wellness push. He claims his biometrics now show he's 7,800% more fit than he was in February ... and he swam over 3 miles in the pool's first two days.

Steve-O also buried a few keepsakes in the concrete -- his handprints, signature, a 1-pound anvil and his medallion marking 18 years of sobriety.

But the wildest addition came courtesy of Johnny Knoxville.

Steve-O says Knoxville had butt plugs made for the "Jackass" cast featuring director Jeff Tremaine's head ... and he wanted one cemented into the pool to honor the franchise and the friendships it gave him.

Problem was, Steve-O had lost his ... so he hit the "Jackass" group chat asking someone to overnight one before the concrete was poured.

Sure enough, two showed up in the mail just in time to make the concrete pour.

Steve-O says the new pool will help him train year-round ... and next month he's planning to run two triathlons in one week, including his first Olympic-distance race.