O'Neal disclosed his decision to call it quits in a short video on tout.com. He also put a link to the video in a Twitter message.
"We did it. Nineteen years, baby," O'Neal said in the video. "I want to thank you very much, and that's why I'm telling you first. I'm about to retire. Love you. Talk to you soon."
The 7-foot-1 O'Neal, 39, won four NBA titles -- three with the Los Angeles Lakers and one with Miami -- was an all-star 15 times and was the league's MVP in 2000.
He also played for Orlando, Phoenix, Cleveland and Boston.
This past season he was plagued by injuries. He was out between Feb. 1 and April 3 with a strained Achilles. Only 5 1/2 minutes into his return, he hurt his right calf and played just 12 minutes in two playoff games in the Eastern Conference semifinal series.
O'Neal ranks fifth all-time in points scored (28,596). He averaged 23.7 points, 10.9 rebounds and 2.3 blocks in 1,207 career games, making 58.2 percent of his field goals but just 52.7 percent of his free throws.
"For 19 seasons, Shaquille O'Neal was literally and figuratively an NBA giant," NBA Commissioner David Stern said. "On behalf of the NBA, its teams, and his millions of fans around the world, I want to thank Shaq for everything he has meant to the league and to the sport of basketball, both on and off the court."