Jackson, who had been barred from team activities, had the attempted armed robbery charges against him dismissed earlier this week after the Knox County district attorney said there wasn't sufficient evidence to proceed.
Jackson's two former teammates, Nu'Keese Richardson and Mike Edwards, still have charges pending from the Nov. 12 incident outside a convenience store in Knoxville.
"As I've said before, we wanted to take our time. These were very serious allegations," Kiffin said. "We wanted to make sure we handled this properly. After an extensive and thorough investigation by the Knoxville Police Department, they have cleared Janzen of any wrongdoing. He will join the team activities today."
Jackson has missed the past three games, including the Memphis game prior to his arrest. He was suspended for that game for what Kiffin said was a violation of team rules. Sources have told ESPN.com that a failed drug test was one of the factors leading to Jackson's suspension.
Prior to that suspension, Jackson had started in every game but the opener. The last game he played in was against South Carolina on Oct. 31, and he had a career-high seven tackles in that game and forced a fumble.
"I would sure hope that it is a wake-up call," Kiffin said. "Even though you're cleared of any wrongdoing, it's just a wake-up call to what the reality is. It's, 'Hey, if I did do something like that, look what could be taken away from me.' "
Chris Low covers the SEC for ESPN.com. Information from The Associated Press was used in this report.