The front door of the Shrine of St. Joseph was set alight with accelerant early on Thursday.
No one was thought to have been injured in the blaze, according to USA Today.
St. Louis Police Chief Sam Dotson said another officer saw the fire at the Catholic church whilst responding to an alarm at the church.
He added that they thought the person that started the fire appeared to know how to get in and out of St Joseph church without being seen.
The fire was started in a similar way to the previous six fires, however this one was in a different part of the city and was in a majority white church.
Previously the fires have been started in the north of the city and at majority black churches.
St. Louis Fire Chief Dennis Jenkerson said "very similar" to the other church fires.
Spokesperson Fire Capt. Garon Mosby said the possibility that the fires are hate crimes for religious or racial reasons "is part of the dynamic" of the investigation, but he stressed the reason for the fires are not yet known.
No one was inside any of the churches when the fires were started.
A reward of $9,000 (almost £6,000) has been offered for any information leading to the arrest of the person responsible for the fires.
