The seventh-placed team in the Pro12 will face Gloucester away in the play-offs for the European Champions Cup.
Gloucester qualified for the play-offs by winning the Challenge Cup final against Edinburgh.
The Scottish side are currently seventh in the Pro12, while Scarlets are one point ahead in sixth, the last automatic qualifying place.
The winner of Gloucester's game against the Pro12 side host the seventh-placed French side in the play-off final.
|Pro12 European qualification|
|Guaranteed: One place each for the top teams from Ireland, Italy, Scotland and Wales|
|Then: The next three highest-placed teams in the table.|
|Plus: The next highest-placed team to enter a play-off with the seventh-placed team in England's Aviva Premiership for the right to play the seventh-highest team in France's Top 14 for a place in the Champions' Cup.|
|The rest: Will play in the European Challenge Cup|
There are two rounds of games left in the Pro12 season, which ends on Saturday, 16 May.
The following Sunday, the seventh-placed Pro12 side will face Gloucester at Kingsholm.
The victor of that fixture will have home advantage against the seventh-ranked club in the Top 14 on Sunday, 31 May.