Hornibrook had been the Badgers' starting quarterback the past three seasons and is entering his fifth year of eligibility.
"Alex informed us of his decision to leave the team earlier today," Wisconsin coach Paul Chryst said. "He contributed to a lot of our recent success and we want to thank him for all he did for our program. We wish him the best of luck."
Hornibrook was 26-6 in three seasons as the Badgers' starter. In nine games this past season, Hornibrook completed 59.5 percent of his passes for 1,532 yards and 13 touchdowns with 11 interceptions. While Wisconsin ranked No. 1 in rush yards for all FBS programs, the Badgers ranked 115 in passing yards.
He posted the news Wednesday on Instagram.
Hornibrook dealt with back and head injuries throughout the 2018 season, giving Jack Coan playing time in an up-and-down season. In his three seasons as the starter, Hornibrook threw for 5,438 yards and 47 touchdowns with 33 interceptions.
Wisconsin signed the No. 1 pocket-passing quarterback in the 2019 class in Graham Mertz, who is ranked No. 21 overall in his class.
Mertz, a 6-foot-3, 205-pound recruit from Blue Valley North High School in Overland Park, Kansas, enrolled early at Wisconsin and now joins Coan, who will be a sophomore in 2019, and Chase Wolf, who signed in the 2018 class.