Federation Space Systems was a shipbuilding entity that built Starfleet vessels for the United Federation of Planets.

The Constellation-class USS Stargazer was build by Federation Space Systems. (TNG: "The Battle", dedication plaque)

The Galaxy-class USS Enterprise-D was built by the Federation Space Systems Division at the Utopia Planitia Fleet Yards in the mid-24th century. (TNG: "Encounter at Farpoint", dedication plaque)

The Federation Space Systems was mentioned on the first iteration of the Enterprise-D dedication plaque. [1]
According to Michael Okuda, the FSS was also mentioned on the Stargazer dedication plaque. [2](X)