Scarlets winger Kristian Phillips scored a fine solo try as the West Wales region ran out 19-13 victors over Racing Metro in Colombes.
With pre-match reports suggesting Rhys Priestland is about to turn his back on lucrative offers to play his rugby outside of Wales, the win in Paris came as another timely boost to the region.
Priestland kicked five out of six attempts at goal and controlled the game well as Scarlets won a tight and occasionally fractious clash between the Pool 4 also-rans, which was ultimately decided by a superb try from Phillips.
Priestland got the Scarlets on the scoreboard as early as the fourth minute with a penalty after Racing had been found guilty of illegal hands in the ruck by referee Luke Pearce, the young English official who was called upon twice in the first half to prevent frequent skirmishes breaking out into full-blown fights.
Mike Phillips then scored his maiden try for Racing against his former club when he burrowed over the Scarlets line following a well-worked lineout move. Jonathan Sexton converted.
Priestland and Sexton exchanged further penalties as the half closed with Racing leading 10-6.
Three minutes into the second period Scarlets had a great opportunity to regain the initiative but left wing Fraser Climo was unable to cleanly gather in Priestland's chip ahead with the line at his mercy.
Scarlets continued to show good attacking intent though, which was backed up by some sterling work by the tight forwards, and they were rewarded when Priestland kicked his third penalty after Racing failed to roll away at the breakdown.
A fourth penalty saw Scarlets take the lead for the second time in the match, however the advantage was short-lived after Pumas star Juan Martin Hernandez was successful with his first attempt from the kicking tee.
Phillips, of the Scarlets, then scored a try worthy of winning any match when he returned a poor clearance kick on the Racing 10-metre line and weaved his way through the a static defence to score the all-important try on 63 minutes.
Pens: Sexton, Hernandez
Try: K Phillips
Pens: Priestland 4
Racing: 15 Benjamin Fall, 14 Juan Imhoff, 13 Henry Chavancy, 12 Juan Martin Hernandez, 11 Marc Andreu, 10 Jonny Sexton, 9 Mike Phillips, 8 Jacques Cronje, 7 Dan Lydiate, 6 Antoine Battut, 5 Fabrice Metz, 4 Karim Ghezal, 3 Brian Mujati, 2 Virgile Lacombe, 1 Julien Brugnaut
Replacements: 16 Jerome Maurouard, 17 Soane Tonga'uiha, 18 Luc Dulcacon, 19 Juandre Kruger, 20 Camille Gerondeau, 21 Maxime Machenaud, 22 Benjamin Dambielle, 23 Alexandre Dumoulin
Scarlets: 15 Aled Thomas, 14 Kristian Phillips, 13 Nick Reynolds, 12 Scott Williams, 11 Frazier Climo, 10 Rhys Priestland, 9 Gareth Davies, 8 Rob McCusker (c), 7 John Barclay, 6 Aaron Shingler, 5 Johan Snyman, 4 Jake Ball, 3 Samson Lee, 2 Emyr Phillips, 1 Rob Evans
Replacements: 16 Kirby Myhill, 17 Shaun Hopkins, 18 Jacobie Adriaanse, 19 Richard Kelly, 20 Josh Turnbull, 21 Rhodri Williams, 22 Gareth Maule, 23 Jordan Williams
Referee: Luke Pearce (Eng)