The university is appealing the ruling.
Trier, who missed the first 19 games of last season after testing positive for a performance-enhancing substance, is averaging 19.6 points for an Arizona team that is considered a Final Four contender. He was on the bench, in street clothes, for the Wildcats' 75-65 overtime victory at Oregon State on Thursday night.
The university said in a statement that Trier participated in an NCAA student-athlete drug screening in late January. The results were confirmed Thursday and "revealed the reappearance of a trace amount of a banned substance."
"The amount detected was miniscule by scientific standards and appears to be a remnant of a substance, which the NCAA agreed, Allonzo had unknowingly ingested in 2016," the school said.
A year ago, Trier tested positive for the PED in the preseason and wasn't cleared until Jan. 20, 2017 -- when tests showed that all traces of the substance had left his system. Trier said that he never "knowingly" took the banned substance.
In a statement to ESPN, Trier's mother, Marcie, said she is "devastated and heartbroken" by the NCAA's decision.
Marcie Trier's statement to ESPN on her son, Allonzo's, suspension by the NCAA: pic.twitter.com/IyNUdZdq93— Jeff Goodman (@GoodmanESPN) February 23, 2018
No. 14 Arizona is 22-6 overall and 12-3 in the Pac-12 as the conference's first-place team. Trier's scoring average is second in the conference behind teammate Deandre Ayton (19.7) entering Thursday.