Pb₂Bi₂GaB₃O₁₁, a new congruent-melting galloborate, which simultaneously contains two types of lone-pair cations and a π-conjugated system in one crystal structure, has been discovered through spontaneous crystallization. Single-crystal X-ray structural analysis shows that Pb₂Bi₂GaB₃O₁₁ crystallizes in the monoclinic space group of C2/c. The parameters of the unit cell are a = 17.318(5) Å, b = 8.868(3) Å, c = 6.779(2) Å, β = 90.238(4)° and Z = 6. The structure of Pb₂Bi₂GaB₃O₁₁ features three kinds of tunnels, circled by cation polyhedra, in two of which isolated BO₃ groups reside. However, the open Bi₄O₄ channel, as the third tunnel formed by the Bi–O polyhedra, is hollow. Pb₂Bi₂GaB₃O₁₁ possesses an experimental band gap of 4.43 eV and an ultraviolet (UV) cut-off edge of about 280 nm, which is proven by its diffuse reflection spectrum. Theoretical calculations have also been executed to evaluate its band structure, partial density of state (PDOS) and birefringence. The results indicate that the title compound possesses a direct band gap with a theoretical value of 3.48 eV, and even indicate that the Pb²⁺ and Bi³⁺ ions are stereochemically active and cooperate with the BO₃ groups, which contributes to a moderate birefringence of about 0.07 at 589 nm for Pb₂Bi₂GaB₃O₁₁.