The decision was announced in a statement from the school on Wednesday.
Players who declare for the draft have until April 16 to withdraw and keep their NCAA eligibility, but Pressey is hiring an agent, essentially sealing his decision.
Pressey, a 5-foot-11 guard from Dallas, averaged 11.9 points, 7.1 assists and 1.8 steals per game in leading the Tigers to a 23-11 record and an appearance in the NCAA tournament, where they lost their opening game to Colorado State. He was all-Southeastern Conference and is Missouri's career in assists with 580. He tied Anthony Peeler for the school's career steals lead with 196.
But Pressey made only 37.6 percent of his field goal attempts and struggled with late-game decision-making and crucial turnovers in losses to UCLA, Louisiana State, Texas A&M, Arkansas, Kentucky and Tennessee. He averaged 3.5 turnovers per game.
Pressey said in the statement that the decision was difficult.
"I wish we could have brought home a national championship banner, but coming to Mizzou was the best decision of my life," he said.
Coach Frank Haith said in the statement that he supports Pressey's decision, though many draft experts project Pressey as a late-second round pick at best.
"It's a bittersweet day because we all care so much about Phil, but I know how much research he and his family have done and I support that 100 percent," Haith said.
Missouri will have an entirely different look in 2013-14 with the departure of Pressey and seniors such as Laurence Bowers, Alex Oriakhi and Keion Bell. Sophomore Jabari Brown will be the Tigers' only returning starter, though senior guard Earnest Ross, who saw significant playing time, will be back. Transfer Jordan Clarkson, who sat out this past season after averaging 16.5 points per game at Tulsa in 2011-12, will be eligible.