The first silver bismuth borate, AgBi ₂B ₅O ₁₁ (silver dibismuth pentaborate), has been prepared via glass crystallization in the Ag ₂O–Bi ₂O ₃–B ₂O ₃ system and characterized by single-crystal X-ray diffraction. Its structure is derived from that of centrosymmetric Bi ₃B ₅O ₁₂ by ordered substitution of one Bi ³⁺ ion for Ag ⁺, which results in the disappearance of the mirror plane and inversion centre. Second harmonic generation (SHG) measurements confirm the acentric crystal structure. It is formed by [Bi ₂B ₅O ₁₁] _∞ layers stretched along c and comprised of vertex-sharing B ₅O ₁₀ and BiO ₃ groups which incorporate the Ag ⁺ cations. The new compound was characterized by thermal analysis, high-temperature powder X-ray diffraction, and vibrational and UV–Vis–NIR (near infrared) spectroscopy. Its thermal expansion is strongly anisotropic due to the presence of rigid B ₅O ₁₀ groups aligned in a parallel manner. The minimal value is observed along their axis [parallel to c, α _c = 3.1 (1) × 10 ⁻⁶ K ⁻¹], while maximal values are observed in the ab plane [α _a = 20.4 (2) and α _b = 7.8 (2) × 10 ⁻⁶ K ⁻¹]. Upon heating, AgBi ₂B ₅O ₁₁ starts to decay above 684 K due to partial reduction of silver; incongruent melting is observed at 861 K. According to density functional theory (DFT) band-structure calculations, the new compound is a semiconductor with an indirect energy gap of 3.57 eV, which agrees with the experimental data (absorption onset at 380 nm).