Diamond, who is 6-foot-6 and 291 pounds, is the 10th-ranked offensive tackle and 94th-ranked overall player in the Class of 2012, according to ESPNU.
His final list also included Arkansas, Michigan and Wisconsin. He made official visits to all four schools, going to Auburn on Dec. 16, Michigan on Jan. 13, Arkansas on Jan. 20 and Wisconsin on Jan. 27.
He said he was torn about his choice until Wednesday night.
"I had mixed emotions about every school," Diamond said. "I liked a few things about every school, but when it came down to it, the people down there at Auburn really separated themselves from everyone else.
"I slept on it, prayed on it, dreamed about it. That feeling came when I made that decision."
One of the most influential people in Diamond's decision was Auburn offensive line coach Jeff Grimes. Diamond was swayed by his coaching style and him as a person.
"Definitely, coach Grimes put it over the top," Diamond said. "The way he conducts things down in the Southeastern Conference is very amazing. The poise he has his for his athletes, the way he teaches, the family guy he is convinced me to come play for him."
Auburn now has 21 players in its 2012 recruiting class.
Diamond made his decision while standing in front of six hats, which in addition to his finalists, included Illinois and Notre Dame. He said Notre Dame was an outside possibility, but Illinois was a decoy hat.
"It was really difficult for Jordan," Simeon coach Dante Culbreath. "I think about three days ago he might have been going to Wisconsin, so I knew it was a difficult decision. It came down to the last days."
Diamond was an ESPNChicago.com all-area selection and helped Simeon to a 10-4 season and a runner-up finish in the Prep Bowl.
At the news conference, Simeon quarterback Robert Gregory also committed to Purdue. Gregory was previously committed to Arkansas, but backed out because he wanted to go to a school where he could play quarterback.
"I made a late decision predicated on a late offer," said Gregory, who also considered Minnesota. "I wanted to play quarterback and not a lot of schools would give me the opportunity to play quarterback. A lot of schools wanted to recruit me as an athlete, and Purdue gave me an opportunity. If it calls for a position change, I'll do that, but they give me the opportunity. They made me feel like I'm at home."
Simeon's Terrick Whiten and Kirk Young also committed to Division II McKendree at the news conference.
Scott Powers covers high school and college sports for ESPNChicago.com and can be reached at firstname.lastname@example.org.