The calf was found on Sunday morning with injuries to its anus and had to be given medical treatment by a vet, Aargau police said in a statement.
Police found a belt in the barn that they believe belongs to the unknown perpetrator.
Anyone with information is asked to call police on 061 836 3737.
It's not the first time zoophilia has hit the headlines in Switzerland.
In 2015 a series of sexual acts on horses were reported in the canton of Jura.
In February of that year one man was found guilty of abuse against animals in the village of Courroux.
But several new cases then occurred later that year.