After swapping Glasgow Warriors for the Scarlets this summer, van der Merwe at last made his debut for the Welsh region and marked it with a two-try showing that maintained their flying start to the season.
The 29-year-old, who scored four tries in as many World Cup games for Canada, doubled up against Leinster while James Davies also dotted down.
Isa Nacewa and James Tracy scored tries for the visitors but it wasn't enough to prevent Scarlets running out 25-14 victors - their fourth win in as many GUINNESS PRO12 games this season.
Only one side lies above Scarlets after four rounds and that is Munster, following their 35-27 triumph over Cardiff Blues.
Ian Keatley shone for the league leaders, grabbing a try and five conversions as Anthony Foley's side recovered from an early ten-point deficit.
Mike Sherry notched a try double, while Andrew Conway and Jordan Coghlan also crossed over to ensure they clinched maximum points.
The Ospreys secured their first win of the GUINNESS PRO12 season but they were made to sweat by a resilient Benetton Treviso side at the Stadio Monigo.
Brendon Leonard, who joined the Welsh region from Treviso's Italian rivals Zebre this summer, was named man-of-the-match while a 15-point haul from his half-back partner Sam Davies made sure of the 25-22 victory.
Simone Favaro scored a debut try for Glasgow Warriors, while a fit-again Alex Dunbar also impressed in their 29-15 win against Newport Gwent Dragons.
Favaro's try, which followed scores from Chris Fusaro, Zander Fagerson and Junior Bulumakau, clinched a try bonus point that sees the Warriors move into the GUINNESS PRO12 top half.
Connacht, meanwhile, collected their third try bonus point of the season - more than any other side - with a 34-15 victory over Zebre.
Indeed, Pat Lam's side had wrapped up that bonus before the break and sit just a point behind table-topping Munster through four rounds.
Ahead of them in third, however, are Edinburgh after Hamish Watson inspired them to a 16-10 victory over Ulster at BT Murrayfield.
TRY OF THE WEEKEND
Footwork was the name of the game across the GUINNESS PRO12 this weekend with plenty of tries - including Connacht's first two - featuring some silky skills on the ground.
Junior Bulumakau's effort, however, was the pick of the bunch as the winger came off the bench to help Glasgow Warriors to a bonus-point win.
Mike Blair's box kick wreaked havoc at the Dragons 22 and, after side-footing the ball towards the try line, the Fijian-born flyer sped along outside the right touchline before winning the race to touch down
BREAK OF THE WEEKEND
Treviso could only manage a losing bonus point at home to Ospreys but James Ambrosini gave the Stadio Monigo support plenty to shout about in the first half.
The fly-half received the ball 30 metres out from the Ospreys line and immediately broke a tackle.
After drifting towards his left, where Treviso boasted an ominous overlap, Ambrosini decided to go it alone, stepping inside between two defenders and out-pacing Sam Davies to cross over.
PLAY OF THE WEEKEND
Cardiff Blues' first try at Munster was a thing of beauty, with the ball spread wide to the right and back again, but their second was arguably more special.
For that, the Welsh side had Gavin Evans to thank after the inside centre showed superb awareness to fool his marker.
When Rhys Patchell fired the ball to Evans by the left corner, his deceptive dummy allowed it to fly through to Richard Smith for an excellent try.
PLAYER OF THE WEEKEND
Omitted from Ireland's Rugby World Cup squad, Ian Keatley showed the men in green what they were missing on Saturday afternoon.
The Munster fly-half was in inspired form at Irish Independent Park - just 24 hours before Ireland were knocked out of the World Cup by Argentina - with a 15-point haul.
That featured a clinically-finished try in the first half and a successful conversion for each of table-topping Munster's five scores against Cardiff Blues.
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