A new rare-earth borate Na3YB8O15 was fabricated by the flux method. The compound crystallizes in the P1̄ group, and shows a three-dimensional network built by [B8O17] fundamental building blocks (FBBs) with [NaO6], [NaO7], and [YO7] polyhedra. The [B8O17] FBBs are composed of five planar BO3 and three BO4 tetrahedra and connected via corner-sharing to construct a unique one-dimensional (1D) [B8O15]∞ infinity chain that is observed for the first time in the Na2O-Y2O3-B2O3 system compound, enriching the structural diversity of rare-earth borate. The title compound exhibits a short ultraviolet (UV) cutoff edge of ∼195 nm and a wide experimental band gap of 5.50 eV. DFT calculations indicate that Na3YB8O15 is a direct band gap compound with a calculated GGA band gap of 5.14 eV, and the band gap is mainly determined by B 2p and O 2p orbitals. The results update the phase diagram of Na2O-Y2O3-B2O3 and enrich the diversity of rare-earth metal borates.