A master chief petty officer is a senior non-commissioned officer in many navies and coast guards, usually above some grade of petty officer.

Master chief petty officer (MCPO) is the ninth (just below the rank of MCPON) enlisted rank (with pay grade E-9) in the United States Navy and United States Coast Guard, just above Senior Chief Petty Officer (SCPO). Master chief petty officers are addressed as "Master Chief (last name)" in colloquial contexts. They constitute the top 1.25% of the enlisted members of the maritime forces.

Prior to 1958, chief petty officer was the highest enlisted rate in both the U.S. Navy and U.S. Coast Guard. This changed on 20 May 1958 with the passage of Public Law 85-422, the Military Pay Act of 1958, which established two new enlisted pay grades of E-8 and E-9 in all five branches of the U.S. Armed Forces. In the Navy and Coast Guard, the new E-8 pay grade was titled Senior Chief Petty Officer and the new E-9 pay grade as Master Chief Petty Officer, with the first selectees promoting to their respective grades in 1959 and 1960.

In the Navy, advancement to master chief petty officer is similar to that of chief petty officer and senior chief petty officer. It carries requirements of time in service, superior evaluation scores, and selection by a board of master chiefs. Similarly, senior chief petty officers and chief petty officers are chosen by selection boards. In the Coast Guard, advancement to master chief petty officer is done by competing in a single annual board. The board ranks all eligible senior chiefs using their career records. Master chief petty officers are then selected monthly from this prioritization list as positions become available.

Petty officers of all grades possess both a rate (the enlisted term for rank) and rating (job, similar to a military occupational specialty (MOS) in other branches). The full title (most commonly used) is a combination of the two. Thus, a master chief petty officer with the rating of fire controlman would properly be called a master chief fire controlman.

Each rating has an official abbreviation, such as FC for fire controlman, FT for fire control technician, and STS for sonar technician, submarines. When combined with the rate abbreviation (MC for master chief without rating), it produces the full rate designation, such as FCCM for fire controlman chief master (more commonly said as Master Chief Fire Controlman). It is not uncommon practice to refer to the master chief by this shorthand in all but the most formal correspondence (such as printing and inscription on awards). Mostly, though, they are simply called "master chief", regardless of rating.

The rate insignia for a master chief is a white eagle with spread wings above three chevrons. The chevrons are topped by a rocker (arc) that goes behind the eagle. Two inverted silver stars (a reference to the stars used on the sleeves of line officers) are placed above the eagle. Between the arc and the top chevron is the specialty mark of the enlisted rating. This is used on the service dress blue, dinner dress blue jacket, and dinner dress white jacket uniforms. On other uniforms, the insignia used for shirt collars and caps is the one that has become universally accepted as the symbol of the chief petty officer. This is a gold foul anchor (though sometimes the word "fouled" is used, the proper term is "foul anchor") superimposed with a silver "USN" (Navy) or a silver shield (Coast Guard). As on the rating badge, this is capped by two five-pointed stars, showing one ray down.

Master chief petty officers are generally considered to be the technical experts in their fields. They serve at sea and ashore in commands of all sizes. Some master chiefs choose to enter the command master chief petty officer program. If selected, a master chief receives additional leadership training and is assigned to a command as the command master chief (CMC). The command master chief is the senior enlisted person at a command and works as a liaison between the commanding officer and the enlisted ranks, serving as the senior enlisted leader. In this capacity, the CMC assists the commanding officer in issues of quality of life, discipline, training, and morale. On submarines, the equivalent of a CMC is called the chief of the boat or "COB". The CMC insignia has a silver star in lieu of the enlisted rating insignia between the rocker and the top chevron.