Dalvos Prime was the inhabited prime planet in its system. This planet was home to a special long-term medical care center.

In 2374, a Federation Magistrate suggested that an adult Molly O'Brien (aged by an incident with a time portal on Golana) be placed there, as a compromise to avoid incarceration for attacking a Tarkalean in Quark's. (DS9: "Time's Orphan")

This planet was only mentioned in dialogue.
According to Star Trek: Star Charts ("United Federation of Planets I"), in 2378, Dalvos Prime was listed as a Federation member.