The arrival of the 29-year-old lock will be a major boost to the Scarlets as they prepare for the new season which includes a difficult Heineken Cup draw.
The transfer of Louw from French side Clermont Auvergne looked to be in jeopardy under new regulations brought in on February 8. But the Scarlets successfully appealed to the Home Office as Louw was signed before the regulations came into force.
Scarlets Chief Executive Stuart Gallacher was pleased to have wrapped the drawn out deal up. He said: "We're delighted and looking forward to Hottie joining us in a few weeks."
The Scarlets will face Heineken Cup holders Toulouse, former champions Wasps and Scottish outfit Edinburgh following last week's draw for the pool stages of next season's competition
When the Wales squad were asked to nominate individuals who played a big part in their early playing days, Wales and Dragons full back and wing Hallam Amos thought of his former Monmouth School rugby coach John Bevan.
Sam Cross and Luke Treharne have been named as co-captains of the Wales Sevens side as head coach Gareth Williams names his 12-man squad to play in the first leg of the HSBC World Series in Dubai next weekend.
Bedwas head coach Steve Law and players Nicky Griffiths and Alun Rees extol the virtues of the close knit club as they prepare to host third place RGC 1404 - who are one place head of them on the Principality Premiership points table - on Saturday.
Many of the Wales squad have said Diolch (thank you) to someone who played an important part in their early playing days by nominating them to be invited to the Wales v South Africa match. One person, Gowerton Comprehensive geography teacher Mr Dean Mason was nominated by two of his former pupils, Dan Biggar and Sam Davies.
The second WRU disability rugby pilot has been delivered, this time in Caerphilly schools, culminating in a festival at the Caerphilly Centre of Sporting Excellence, Ystrad Mynach tomorrow. Plans are in place to set up a Disability Rugby Club in Newbridge to provide long-term playing opportunities.
WRU TV: Hear from inside the Wales camp as Neil Jenkins discusses this weekend's clash with South Africa and how the squad are looking to put in a strong performance to round of the 2016 Under Armour Series