Harlequins signed Irne Herbst, second row, from Benetton in Italy for next season.
The South African-born South African, 29, has been with the United Rugby Championship club for the past five years.
He said, “I'm thrilled to have signed with Harlequins”, to the club's reporters website.
It is an amazing opportunity to join such a prestigious English rugby team and be a Premiership champion.
Quins are in third place in Premiership. To guarantee their place in the end of season play-offs, they will need to win against Exeter or Gloucester in their final two games.