The banned substance furosemide was found in a urine sample provided by the Cameroon international in October last year.
The year-long ban comes into effect today, February 5, and means Onana is suspended from all football activities at both national and international level.
Ajax claim the 24-year-old took the substance “unwittingly” after feeling unwell and are planning to appeal the decision to the Court of Arbitration for Sport.
The club statement said: “On the morning of 30 October Onana did not feel well. He wanted to take a tablet. Unwittingly, however, he took Lasimac, a drug that his wife had previously been prescribed.
“Onana made a mistake with the product and mistakenly took his wife’s medicine, which ultimately led to UEFA’s action against the goalkeeper. The football association’s disciplinary body ruled that he did not intend to cheat.
“However, the European Football Association believes that, according to the prevailing anti-doping rules, an athlete has a duty at all times to ensure that no prohibited substances enter the body.”