Robertson, a native of Savannah, Georgia, won't have to sit out one season under normal NCAA transfer rules. He chose the Bulldogs over Alabama, Texas and West Virginia after leaving the Golden Bears following his sophomore season.
"I'm excited for Demetris. He's a great kid and he's blending in well with our team," coach Kirby Smart said. "He played with toughness and he competes every day. He's got good speed, he's bright and he's picked up the offense. We've just let him go out and play, and he has picked things up.
"There's a lot of competition in the receivers room, and I'm proud of the way Demetris has competed. He needs to keep working and improving and help us where he can."
He suffered a season-ending leg injury two games into last season and announced in June that he was leaving Cal for "personal reasons." He had seven catches for 70 yards in 2017.
The 6-foot, 190-pound Robertson had 50 catches for 767 yards with seven touchdowns at Cal in 2016, breaking freshmen receiving records for catches and yards previously held by Keenan Allen and DeSean Jackson, respectively.
Robertson, also a member of Cal's track team, was one of the fastest players in the 2016 recruiting class and was ranked the No. 6 receiver nationally and No. 8 player overall in the state of Georgia in the ESPN 300.
He'll join a Georgia offense that lost star tailbacks Nick Chubb and Sony Michel and leading receiver Javon Wims to the NFL. But the Bulldogs return quarterback Jake Fromm and receivers Terry Godwin, Mecole Hardman and Riley Ridley.
The Bulldogs, the defending SEC champions, are ranked No. 4 in the preseason coaches' poll and open the season against FCS foe Austin Peay on Sept. 1.
ESPN's Chris Low contributed to this report.