Part of Whitton Road was taped off by police while emergency services investigate with the incident.
The Metropolitan Police believe two people on a moped discharged a fire extinguisher at the bus.
The driver of the 281 bus was treated at the scene but police say his injuries are not life threatening or life changing.
Whitton Road was closed in both directions between the junction of Pownall Road and Cross Lances Road / Heath Road.
Bus routes 281 and H20 were placed on diversion.
Police said they were called at 11.01am on Friday (August 11) to reports of a powdered substance being thrown at a bus in Whitton Road.
Officers, London Ambulance Service and London Fire Brigade were in attendance.
A statement from the Met Police said: "At this early stage, it is believed a fire extinguisher was discharged by two people on a moped.
"The bus has been evacuated as a precaution."
A London Ambulance Service spokesperson said: “We were called at 10.59am to reports of an incident on a bus on Whitton Road, Hounslow.
“We sent members of our Hazardous Area Response Team and an Incident Response Officer.
“We treated a man for minor injuries at the scene, but no one was taken to hospital.”
No arrests have been made.
Police enquiries continue.
Passengers on the bus, witnesses and anyone with information are asked to call Hounslow CID on 101 or Tweet @MetCC.
Alternatively information can also be reported anonymously to Crimestoppers online on 0800 555 11.
Follow our live blog for further information as events unfold.
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