Viviani, 30, who plans to defend his 2016 omnium track gold medal at the Tokyo Games next year, has a stage win at each of the three Grand Tours, including a victory in Nancy in the Tour de France last month.
"I noticed when talking to manager Cedric Vasseur and sporting director Roberto Damiani how much Cofidis wanted to sign me and they presented an ambitious project to me," he said.
"I really appreciate their wish and drive to create a dynamic around me and the confidence they have shown in me with the arrival of my friend Fabio.
"Even if I won a stage this year I really want to do more at the Tour de France next year."
Experienced lead-out man and fellow Italian Sabatini, 34, has raced in 20 Grand Tour in a 13-year professional career.
The length of the pair's contracts were not specified.