The 6-foot-8 sophomore, who averaged 14 points and 8.4 rebounds per game last season, tweeted the news from his account on Monday afternoon.
Been on my mind heavy but I love the feeling of being a Louisville cardinal an I will be wearing this for the next yr pic.twitter.com/xuOGr4nsc3
Louisville spokesman Kenny Klein confirmed to ESPN that it is Harrell's Twitter account, and Cardinals coach Rick Pitino said he was "very excited to say the least" to have his big man back in the fold.
"He and Wayne [Blackshear] will be co-captains next season," Pitino said. "I think he wants to come back and be a top-eight pick."
NBA executives told ESPN that Harrell would have likely gone somewhere in the 15-25 range if he had elected to leave school and declare for this year's NBA draft.
Harrell gives the Cardinals a proven big man to go along with plenty of perimeter options. Although Louisville lost Russ Smith and Luke Hancock, the Cardinals return Chris Jones and Terry Rozier and will bring in wing Shaqquan Aaron (ESPN, No. 33) and point guard Quentin Snider (ESPN, No. 40).