Corey Anderson's loss is Luke Ronchi's gain with the New Zealand wicketkeeper/batsman signed to play five domestic Twenty20 fixtures for Somerset.
The English county urgently sought a replacement after Anderson's England tour was ended by a back injury suffered during the first test at Lord's.
"Luke is a quality player with plenty of international experience," said Somerset director of cricket Matthew Maynard.
"He is an excellent wicket-keeper, an explosive batsman, and I am sure our members and supporters will enjoy the brand of cricket that he plays."
The 34-year-old will be available to play his first match against Middlesex next week after the conclusion of New Zealand's tour.