The South African-born star - who is now English qualified - joined Rotherham in August 2005 from the University of the Free State side Shimlas, Bloemfontein, before going onto make 87 appearances, including 38 tries, over his four seasons with the Titans.
The 28-year-old then took his try scoring exploits to London Welsh during the 2009/10 season, scoring 14 tries in 26 league appearances. In total, Claassens scored 19 tries in 34 appearances during the 2009/10 campaign.
Before joining Rotherham Titans, Claassens, the brother of Bath's South African international scrum half Michael, won representative honours with Free State at Under-19 and Under-21 level, as well as playing for the Free State Cheetahs and gaining Currie Cup experience with Griquas Rugby.
"Errie impressed us at the very start of this season when we played London Welsh at Old Deer Park when he had a very good game," said Head Coach Richard Hill. "He can play as a full back or wing, but he has been signed and will come to Sixways to be a full back.
"Errie has a strong left foot, which is an important asset to have in the side, and a lovely step and counter attack. His first instinct is to run and he is certainly an attacking full back.
"His attitude is outstanding and he is very much a sponge who will absorb everything you say. He sees this move as a really good opportunity to continue to improve as a player and we look forward to welcoming him in the summer."