A merchant navy or merchant marine is the fleet of merchant vessels that are registered in a country, which varies in capacity. Seafarers on merchant vessels, who hold various military-like ranks and responsibilities and are sometimes members of various maritime trade unions, are required by the International Convention on Standards of Training, Certification and Watchkeeping for Seafarers to carry Merchant Mariner's Documents. King George V bestowed the title of the "Merchant Navy" on the British merchant shipping fleets following their service in the First World War, however a number of other nations have since adopted the title. The following is a partial list of the merchant navies or merchant marines of various countries. In many countries the fleet's proper name is simply the capitalized version of the common noun.

The British Merchant Navy comprises the British merchant ships that transport cargo and people during time of peace and war.

For long periods of the last millennium, the Merchant Navy had the largest merchant fleet in the world, but it has slipped down the rankings. Today, there are 429 ships of 1,000 Gross Tonnes (GT) or over, making a total of 9,181,284 GT (9,566,275  tons deadweight (DWT)). These are split into the following types: bulk carrier 18, cargo 55, chemical tanker 48, container 134, liquefied gas 11, passenger 12, passenger/cargo 64, petroleum tanker 40, refrigerated cargo 19, roll on/roll off 25, vehicle carrier 3. There are also 446 ships registered in other countries, and 202 foreign-owned ships registered in the UK. (2005 CIA estimate)