Beth Delta I was the first planet in the Beth Delta planetary system. It was an inhabited planet. One of its settlements was named New Manhattan.
Paul Stubbs wanted to take Deanna Troi to this planet, to show her the city of New Manhattan "as you've never seen it" while drinking some champagne. (TNG: "Evolution")
According to the Star Trek: Star Charts, on the chart "United Federation of Planets I", Beth Delta I was listed as a member planet of the Federation in 2378.