Fire crews from Dalkeith and Newcraighall rushed to the blaze at Tesco Hardengreen, after reports were received at around 4am.
They said they arrived to find the car wash and an industrial wheelie bin alight.
A team of 10 firefighters equipped with hose reel and breathing apparatus took around 20 minutes to extinguish the blaze and make the area safe.
The petrol station was cordoned off while police investigated. There were no reports of injuries.
A spokesman said: “Police in Midlothian were called to Bonnyrigg Road, Eskbank, around 4am on Sunday 16 August following a report of a car wash on fire.
“The fire is currently being treated as suspicious and officers are eager to trace five males seen in the area at the time, who are described as being in their early teens and wearing blue/black north face style jackets. One of the group had ginger hair and was wearing a white and blue hooded top.
“Anyone with information is asked to contact Police Scotland on 101 or contact Crimestoppers anonymously on 0800 555111.”