Frustrated at falling down the pecking order in his second season with the Norwegian champions, the attack-minded left-sided raider will use the move as a platform to reignite his challenge for a place in Australia’s World Cup squad.
Lack of match minutes in Norway has hindered Gersbach in his quest for regular call ups for his country, with Aziz Behich and injury-prone Brad Smith his main obstacles in pursuit of a Socceroos starting spot.
He is expected to make the move permanent at the end of the season, if both he and the club - lying 15th on the ladder - hit it off.
READ MORE
Victory reject Milligan transfer bid
The deal was ratified after Gersbach completed a medical on Monday.
He was last week linked with a switch to Championship outfit Barnsley, but when that fell through Lens pounced on the 20 year old.
He started just nine of Rosenborg's 30 Eliteserien League games as they stormed to the Norwegian title, his fourth trophy since signing in 2016 from Sydney FC.
He made six more league appearances off the bench.
Gersbach could make his debut in the French Cup against Saint-Brieuc on Wednesday (AEDT).