Killing it in water gets you a sick place on land ... just ask Michael Phelps, who dropped $2.5 mil on a HUGE mansion in Scottsdale, Arizona ... complete with a sick pool (of course).
TMZ Sports has learned ... Phelps scooped the place up in Dec. 2015 -- right after construction was completed on the 6,010 square foot pad.
It's super private -- with 5 bedrooms and 6.5 baths ... and a pool that looks like it's straight out of a 5 star resort.
Phelps had been training at Arizona State University in nearby Tempe -- and has said he plans to help coach the college swim team now that his Olympic career is over.
The 31-year-old reportedly sold his home in Baltimore -- so, it looks like he's putting down roots in AZ to raise his new baby boy with his fiancee, Nicole Johnson.
To the 23 time victor, go the spoils!