Fly-half Tim Swiel will rejoin Harlequins from Super Rugby side Sharks on a permanent contract this summer.
The 21-year-old England-born South African spent four months on loan with Quins earlier this season, playing 10 times between October and February.
"Tim made a real impact in incredibly trying circumstances," said director of rugby Conor O'Shea.
"He is a young player with a huge amount of potential and is able to cope with anything thrown at him."
Swiel, who is England qualified having been born in Taunton, joined Harlequins as cover for the injured Ben Botica.
"The decision wasn't a tricky one due to the fact I had such a great time on my loan spell," he said.
Harlequins have not disclosed the length of Swiel's contract.