"We are delighted to announce Hottie will be joining us in the summer," Scarlets chief executive Stuart Gallacher told the club's website. Louw said the Llanelli club's reputation helped attract him to Stradey Park.
"I'm looking forward to joining the Scarlets," said the South African. "The name of Llanelli is known throughout the rugby playing world and I'm looking forward to putting on the famous shirt."
His recruitment follows the Scarlets' signings of All Black prop Dave Hewett and Welsh-qualified New Zealand lock David Duley.