Ymer said he had been charged with a potential anti-doping rule violation in January 2022 for missing three out-of-competition test attempts in a 12-month period.
The 24-year-old said he had fought the charges in a hearing and was cleared by an independent tribunal in June 2022 but that the ITF had appealed the decision.
The CAS said the ITF had requested a two-year period of ineligibility but the panel partially upheld the appeal to impose an 18-month suspension.
"Having already been cleared once, and wholeheartedly standing by the fact that I do not feel that the third offence was committed, I find their decision to try me again and subsequently find me guilty, unfair," Ymer wrote on Twitter.
"On top of that, I find it difficult to comprehend that they found an 18 month suspension to be a just punishment.
"I do not believe I broke those rules and my conscience is clear with God as my witness."
The CAS said Ymer accepted the first two whereabouts failures on April 22 and August 10, 2021, but contested the third on November 7, which was blamed on miscommunication with his agent.
"The first-instance tribunal found that... the Doping Control Officer did all that was required of him to locate the player, but that no negligence could be attributed to the player or his agent," the CAS said.
"The player was accommodated in a different hotel than the one he had requested when making his reservation, but the player's agent (who updated the player's whereabouts on his behalf) did not receive news of that change and therefore did not make the necessary update on that occasion."
The CAS panel concluded Ymer had failed in ensuring his compliance with the anti-doping regulations by omitting to verify his whereabouts filing and by assuming it would be corrected by his agent or by the tennis authorities.
Ymer is ranked number 51 in the world and reached the third round at Wimbledon after upsetting ninth seed Taylor Fritz earlier this month.Â
