Delfzijl (; Gronings: Delfsiel) is a city and municipality with a population of 25,651 in the province of Groningen in the northeast of the Netherlands. Delfzijl was a sluice between the Delf and the Ems, which became fortified settlement in the 16th century. The fortifications were removed in the late 19th century. Delfzijl is the fifth largest seaport in the Netherlands, and the largest port in the North East of the country.