Starbase 621 is a Federation starbase.
The Enterprise-D was supposed to visit Starbase 621 in 2370, but delayed this mission for a few days to help in reparing the environmental controls of the Caldos colony. (TNG: "Sub Rosa")
Gul Dukat was being transported by the USS Honshu to Starbase 621 for the beginning of his war crimes hearing when the ship was destroyed and Dukat fled. (DS9: "Waltz")