Middlesex is a city located in the New Jersey region of the United States. During the most recent census, Middlesex had a population of 13,635. Middlesex is the 123rd most poplulated city in the New Jersey region, the 3,114th most populated city in the United States and the 17,594th most populated city in the world. Middlesex is in the America/New York timezone which is 4 hours behind UTC. Middlesex adjusts the hour with daylight savings time during the winter months.