In the Challenge Cup, former Russia coach Kingsley Jones will see Newport Gwent Dragons' become the first Welsh team to face Russian club Enisei STM.
Dragons also face Worcester and Brive, while Ospreys will take on Grenoble, Newcastle Falcons and Lyon.
Cardiff Blues have west country rivals Bath and Bristol to contend with, along with Pau.
Scarlets will be Wales' only team in Europe's top tier next season after finishing fifth in last term's Pro12.
The west Wales region's draw will pit Wales full-back rivals Liam Williams and Toulon's Leigh Halfpenny against each other.
Scarlets head coach Wayne Pivac said: "On the face of it it's an extremely tough challenge for us.
"Toulon have a fantastic record in Europe winning finishing champions three times in the last four years.
"Saracens, as Premiership champions, have set the benchmark over the last 12 months with a lot of their players in the England squad.
"Sale will also be a tough opposition, making it a tough pool on the whole."
Ospreys will spend their first campaign in the second tier, a level at which Cardiff Blues and Dragons are more accustomed to playing.
Bristol's Gavin Henson will come up against former clubs Blues and Bath in the Challenge Cup.
Pool 1: Racing 92, Glasgow Warriors, Leicester Tigers, Munster Rugby
Pool 2: Connacht Rugby, Wasps, Toulouse, Zebre
Pool 3: Saracens, RC Toulon, Scarlets, Sale Sharks.
Pool 4: Leinster Rugby, Montpellier, Northampton Saints, Castres Olympique
Pool 5: Exeter Chiefs, ASM Clermont Auvergne, Ulster Rugby, Bordeaux-Begles
Pool 1: La Rochelle, Gloucester, Treviso, Bayonne.
Pool 2: Grenoble, Ospreys, Newcastle and Lyon.
Pool 3: Enisei STM, Worcester, Dragons, Brive.
Pool 4: Bath, Bristol, Pau, Cardiff Blues.
Pool 5: Harlequins, Edinburgh, Stade Francais, Timisoara.
Scarlets will face holders Saracens, ex-winners Toulon and Sale Sharks in the European Champions Cup in 2016-17.