Starbase 621 was a Federation starbase administered by Starfleet.
The USS Enterprise-D was supposed to visit Starbase 621 in 2370, but delayed this mission for a few days to help in repairing the weather modification network 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")
This starbase was only mentioned in dialogue.