The former major leaguer will move over from his current role as a special assistant in the baseball operations department.
Roberts said he developed a rapport and mutual respect with manager Bud Black, "and his offer to me was huge."
"Ultimately it was too good of an opportunity to pass up," Roberts said. "I look at the past season and how much fun I had with the players and working with staff. It's an opportunity to be more hands on. It's exciting."
During last spring training, Roberts worked with the Padres on their baserunning techniques and worked with the outfielders.
Roberts said he'll bring to his job "just the knowledge of playing and also being on the bases, just relating to these guys. I've lived it, I've had good times, I've had tough times. I just want to bring the experience as well as the energy and passion that I have for the game. I'd like to believe that I can convey that to the players and I'd like to think they'd be better for it."
Roberts has recently battled Hodgkin's lymphoma, undergoing chemotherapy and then radiation treatments.
He believes he has it beaten.
"All my scans are clear," he said.
He played 10 seasons in the majors for Cleveland, the Los Angeles Dodgers, Boston, San Diego and San Francisco. The speedy outfielder retired after 2008 and worked in television last year for the Red Sox.
Roberts was a career .266 hitter with 243 steals.
He replaces Rick Renteria, who will become the bench coach. Former bench coach Ted Simmons is leaving to pursue a managing job or return to scouting.