Pentarus III was the third planet of the Pentarus planetary system. This system was located in the Alpha Quadrant. It had a "barely Class M" moon named Lambda Paz. (TNG: "Final Mission")
Pentarus, the primary of the Pentarus system, was on a star chart that was seen in Star Trek Generations. The chart depicted the location of planets, stars, and star systems in the Alpha Quadrant. Based on the dimensions of the circles used to mark locations on the star chart seen in "Generations", this star was located relatively near to the USS Enterprise-D.
According to the Star Trek: Star Charts, on the chart United Federation of Planets I, Pentarus III, by 2378, had become a Federation member.