The 20-year-old was provisionally suspended in April after testing positive for a metabolite of trenbolone in March.
But an independent arbitrator ruled that it was more likely than not Knighton digested the banned substance when he ate some contaminated meat.
Usada (United States Anti-Doping Agency) said, despite testing positive, there was no fault or negligence on the athlete’s part.
Travis Tygart, the Usada chief executive, said “justice was served”.
Trenbolone is a known livestock growth promoter that is used legally in beef cattle produced in, and exported to, the United States.
The decision comes before the US track and field Olympic trials begin on 21 June in Oregon.
Knighton became the youngest male since Jim Ryun in 1964 to make the US Olympic team when he qualified for the Tokyo Games in 2021 – going on to finish fourth in the 200m final at the age of 17.
