Nouns ending in -at are masculine.
The suffix often marks a status or condition.
Is commissariat masculine or feminine?
lecommissariat
The rule behind it
100% of -at nouns are masculine
In context
Comment puis-je me rendre au commissariat de police ?
How can I get to the police station?
Je cherche le commissariat de police. Pouvez-vous m'aider ?
I'm looking for the police station. Can you help me?
Les policiers jouaient aux échecs au commissariat.
The police officers were playing chess at the police station.