From the Latin merula, meaning "blackbird." The blackbird is said to be a symbol of a naive person. Also, according to different local traditions, it means the opposite, sly or shrewd.

By another account, Merlo is habitational place-name, which is a type of hereditary surname, and is derived from the name of the place in which the first bearer lived. Merlo is derived from is Morlaix, in Brittany, a peninsula in the northwest of France. Formerly known as Armorica, a possession of the Roman Empire, this land consists of a plateau with a deeply indented coast and is broken by hills in the west.

Spelling variations include: Marlowe, Marloe, Marleau, Marlow and others. First found in Cheshire where they were seated from very early times and were granted lands by Duke William of Normandy, their liege Lord, for their distinguished assistance at the Battle of Hastings in 1066 A.D.

There are 5,775 people in Italy with the Merlo surname.