F is the service designation for trains on the ring line of Copenhagen's S-train network.

Trains run every 5 minutes between about 7:00 and 19:00, Monday through Friday, and every 10 minutes on weekends, early mornings and late evenings/nights.

In 2028 this line is planned to become a pilot project for driverless operation of the S-train system, becoming fully automated in 2029.

The ring line also has a history of supplementary services with other designations than F. At some times the variant service letters have been used to indicate which trains continued from Hellerup to Klampenborg.

In other periods it has simply been because then-prevalent principles mandated that a single service letter such as F could not be used for more than exactly 3 trains an hour, so in parts of the day when more trains ran, extra services had to be invented.