Domenico Dolce and Stefano Gabbana were found guilty of failing to declare 1 billion euros ($1.3 billion) in income and were both sentenced to a year and eight months in jail.
The designers have denied the charges.
The two men behind the famous Dolce & Gabbana luxury brand were originally cleared, but a superior court later said they could be charged.
During the case, prosecutors said they had evaded 416 million euros each and another 200 million euros through a Luxembourg shell company called Gado.
The holding company was sold well below market value, said prosecutors.
The investigation into the company by the Italian authorities started about six years ago, part of a government plan to stop tax avoidance in Italy.
The pair has not yet made a public comment about the verdict.