The Spaniard and the Canadian made contact at Turn 1 on lap 13 of the race as Sainz emerged from the pits, forcing both cars to retire and triggering a safety car period.
Both drivers blamed each other for the crash and were summoned to see the stewards after the race.
The stewards found Sainz was at fault for what they labelled a "very optimistic attempt" at passing Stroll and the Spanish driver will drop three places on the grid in the next race in Russia.
Carlos Sainz of Scuderia Toro Rosso and Spain during the Bahrain Formula One Grand Prix at Bahrain International Circuit on April 16, 2017 in Bahrain, Bahrain.
Image credit: Getty Images
He was also awarded two penalty points for a total of seven over the past 12 months.
"The stewards heard from Carlos Sainz, the driver of car 55, Lance Stroll, the driver of car 18 and the team representatives, reviewed the video evidence which showed that car 18 was on the normal racing line, car 55 left the pitlane and made a very optimistic attempt to pass car 18 into the corner," said the verdict on the incident.
"The stewards decided that the driver of car 55 was predominately to blame for causing the collision in violation of Article 27.4 of the FIA Formula 1 Sporting Regulations.
"The competitor is reminded that, in accordance with Article 12.2.4 of the FIA International Sporting Code and Article 38.3 g) of the FIA Formula 1 Sporting regulations, the above penalty is not susceptible to appeal."