Delfzijl (Dutch pronunciation: [dɛlfˈsɛil]; 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.
The name Delfzijl means 'sluice of the Delf'. The Delf was a canal connecting the rivers Fivel and Ems, and is now part of the Damsterdiep. The Dutch verb delven means 'to dig' and the Dutch noun zijl means 'water outlet' or 'sluice'.
Delfzijl was established at the location where three sluices (Dutch: zijlen) were connected. In 1317, a sluice was built in the Delf, a canal which is now part of the Damsterdiep. Delfzijl has been the main sea port of Groningen since medieval times.