Andre Onana has been banned from football for a year by UEFA after failing a drugs test.
The goalkeeper was subject to an "out of competition" check on October 30.
The substance Furosemide was found in his urine with his suspension beginning immediately and applying to both club and international football.
Back in October Onana had been feeling unwell and unknowingly took Lasimac, a drug that his wife had previously been prescribed, to try and ease the pain.
The disciplinary panel has accepted that there was no intent from Onana to cheat or gain any advantage.
However, UEFA has stated on the basis of the applicable anti-doping rules, that it is down to the individual to ensure that no banned substances are taken.
Ajax and Onana will appeal the decision with the Court of Arbitration for Sport (CAS).
Club chief Edwin van der Sar said: “We explicitly renounce performance-enhancing drugs, we obviously stand for a clean sport.
"This is a terrible setback, for Andre himself but certainly also for us as a club.
"Andre is a top goalkeeper, who has proven his worth for Ajax for years and is very popular with the fans.
"We had hoped for a conditional suspension or for a suspension much shorter than these twelve months, because it was arguably not intended to strengthen his body and thus improve his performance. ”
The 24-year-old shot stopper has been a regular at Ajax since joining them in 2016.
He has played 30 plus games in each of the past four seasons and remains the club's No 1.
Onana helped the Dutch side win a league and cup double in 2019 while also being part of the team that reached the semi-finals of the Champions League.
The Cameroon international has previously been linked with a Premier League move.
