The NCAA deadline for players to withdraw from the NBA draft and maintain college eligibility is Wednesday.
Agbaji, a 6-foot-6 wing, was named All-Big 12 honorable mention for each of the past two years, averaging 14.1 points, 3.6 rebounds, 1.9 assists and 1.1 steals in 34 minutes per game, shooting 37% from 3-point range last season.
"I cannot thank my family enough, the NBA teams, my KU coaches and teammates, as well as everyone who supported me in this process," he tweeted. "Now let's get to work! Rock Chalk!"
His return is a major boon to the Jayhawks, as he projects to be their best defender and a major offensive cog for the team, which is ranked No. 4 in Jeff Borzello's Way-Too-Early Top 25.
Two-time first-team All-Pac-12 performer Remy Martin, who starred for Arizona State last season but announced he was transferring to Kansas if he withdrew from the draft, joined Agbaji in opting to help the Jayhawks chase a sixth national championship.
The duo joined sophomore teammate Jalen Wilson in getting feedback from pro scouts before ultimately deciding to head back to school in Lawrence.
"The main objective was to learn what I needed to do to get better and try to be the best player I can be,'' said Martin, who led the Pac-12 in scoring at 19.1 points per game last season. "It was pretty easy when it came to Kansas,'' he said, "playing for such a historic program and a Hall of Fame coach [in Bill Self]. I want to be a part of it. I want to be part of the brotherhood and help the team win as much as I can.''
In addition to Agbaji and Wilson, Kansas returns David McCormack and Christian Braun from last season's starting five. The Jayhawks also added veterans Joseph Yesufu from Drake and Jalen Coleman-Lands from Iowa State via the transfer portal.
Agbaji, the No. 67 prospect in the ESPN 100, will go into next season as a potential first-round pick and Big 12 Player of the Year candidate. He is one of the best defenders in the college game, standing 6-foot-6 with a 6-10 wingspan and a strong 214-pound frame. He improved his perimeter shooting and could very well make another jump next season, as he turned 21 in April and has been on an upward trajectory in each of the past three years.
Jonathan Givony is an NBA Draft expert and the founder and co-owner of DraftExpress.com, a private scouting and analytics service utilized by NBA, NCAA and International teams.
The Associated Press contributed to this report.