The decision caps a whirlwind season in which Olynyk went from afterthought to All-America selection and led Gonzaga to the top spot on The Associated Press poll for the first time in school history.
The 7-foot forward said he made the final decision Thursday night and informed coaches and teammates. He said the big factor was his desire to play in the NBA.
"In the end, it's something you've dreamed about since you were a little kid," Olynyk said Friday, which was his 22nd birthday
Olynyk, a native of Canada, averaged 17.8 points and 7.3 rebounds while shooting 63 percent from the field. He led Gonzaga to a 32-3 season than ended when the top-seeded Bulldogs were upset by Wichita State in the third round of the NCAA tournament.
"We are very proud of what Kelly was able to accomplish at Gonzaga both on the court and in the classroom," coach Mark Few said in a news release. "We wish him all the best in his next challenge."
Olynyk was a reserve player his first two seasons, then red-shirted after his sophomore year to work on his body and his game. Not many people expected him to put together an All-America season for what has been called the best Gonzaga team in program history.
"Nine months ago this wasn't what I envisioned for today," Olynyk said. "It came quicker than I expected and I've got to capitalize."
Olynyk spent most of his childhood in Toronto and said he would love to play for the Raptors. He has been told he might be drafted as high as the middle of the first round.
"It's all speculation," he said.
Olynyk became the first Gonzaga player since Austin Daye to leave early for the NBA. Daye came out following his sophomore year and was a first-round pick by Detroit.
Olynyk started just four games his first two seasons, and averaged 5.8 points per game as a sophomore. He red-shirted after it became apparent he would get little playing time behind future NBA player Robert Sacre during the 2011-12 season.
Olynyk graduated with an accounting degree in December. He has also finished his first semester in Gonzaga's MBA program.
He has not hired an agent yet and is not sure where he will train for the NBA draft.