Bottas' fastest time was a 1:30.937, nearly half a second clear of teammate Lewis Hamilton with a 1:36.128.
The Briton had an incident during the session where he ran over the bollard at Turn 8, and when he rejoined the track Max Verstappen had to take avoiding action. The incident is being investigated by the stewards.
Lando Norris was fifth fastest for McLaren, only 0.945s slower than Bottas but more importantly a tenth-and-a-half quicker than Max Verstappen in the Red Bull.
Norris' teammate Carlos Sainz split the two Red Bull's in sixth place, with Pierre Gasly only 0.015s behind.
Kevin Magnussen rounds out the top ten, with his laptime of a 1.32.789 much stronger than his teammate Grosjean, who struggled with his tyres throughout the session.
Renault did not have a strong session for either of their drivers, as both were outside the top ten. Nico Hulkenberg had a spin in the closing stages of the session, with both cars struggling on their tyres.
Racing Point struggled yet again, and after another tough practice session it became clear that their car is not suited to the colourful Paul Ricard circuit.
George Russell was once again faster than his teammate Robert Kubica. However, both Williams were still far off the pace of the field, with the Brit half a second away from Lance Stroll in the Racing Point.
Bahrain International Circuit - Winter testing