Emergency services were called after locals noticed smoke coming from the building in West Dunbartonshire, Scotland at around 8pm on Saturday.
The fire was extinguished by a fire crew inside the church.
Detectives have launched an investigation and are urging anyone with information to get in touch.
Inspector Allan Orr said: "Thankfully there was no-one in the building and no-one injured as a result of the fire or the smoke.
"However, had the two members of the public not noticed the smoke, the fire could have spread and the consequences much worse."
A Scottish Fire and Rescue spokeswoman said: "We received a call on Saturday at 7.58pm at a derelict church on Main Street in Alexandria.
"There was a small fire on the ground floor which was put out by our crew by 8.29pm."
Police ask that anyone who may have seen anything suspicious to contact them on 101.
