Two new alkali–metal bismuth iodates KBi(IO₃)₃(OH) (1) and NaBi(IO₃)₄ (2) have been synthesized by a hydrothermal method via tuning of alkali–metal ions. Compound 1 crystallizes in the space group P and features a [Bi(IO₃)₃(OH)]_n chain with eight-membered Bi–O–I–O–Bi–O–I–O rings. The [Bi(IO₃)₃(OH)]_n chain consists of paddlewheel-like dimeric Bi₂(OH)₂(IO₃)₄ units linked by IO₃ pairs. Compound 2 in the polar space group Cc features a [Bi(IO₃)₄]⁻ double layer with Bi₄I₄ eight-membered rings (8-MRs). NaBi(IO₃)₄ exhibits a strong second harmonic generation (SHG) response of ∼5.0 × KDP (KH₂PO₄) and a wide transmittance window (0.4–12 μm), and structure analyses and theoretical calculations reveal that polarization results mainly from the proper alignment of the IO₃ groups. The calculated electronic structure based on the DFT method indicates that the BiO_n and IO₃ groups dominate their optical properties.