Fire crews battle a blaze at a bank in Woodstock (Supplied: Woodstock fire department)

Woodstock police are investigating at suspicious fire that caused $500,000 in damage at a bank on Thursday night.

A passerby noticed flames through the window of the TD Bank on Dundas Street at around 10:45 p.m.

The fire was mostly contained to the ATM vestibule, officials say.

No one was inside the bank at the time.

The cause is under investigation.