A Petty Officer is a noncommissioned officer or equivalent in many navies.
There are two Petty Officer ranks in the Canadian Navy. Petty Officer 2nd Class (PO2) (Maître de deuxième classe in French) is equivalent to a Sergeant and Petty Officer 1st Class (PO1) (Maître de première classe) is equivalent to a Warrant Officer.
In the Royal Navy, the rank of Petty Officer comes above that of Leading Rating and below that of Chief Petty Officer. It is the equivalent of Sergeant in the Royal Marines, British Army and Royal Air Force. Petty Officer is the lowest of the Senior Rating grades. Petty Officers wear officer-style formal uniforms.
British Ratings and Other Ranks
The title of Petty Officer in the United States Navy and United States Coast Guard has three separate grades (3rd class through 1st class). A Petty Officer Third Class is equivalent to a Corporal in the United States Army and Marine Corps and a Senior Airman in the United States Air Force. A Petty Officer Second Class is equivalent to a Sergeant in the United States Army and Marine Corps, and a Staff Sergeant in the United States Air Force. A Petty Officer First Class is equivalent to a Staff Sergeant in the United States Army and Marine Corps and a Technical Sergeant in the United States Air Force.