Glasses with a high content of niobium oxide are of significant interest for electro-optics and nonlinear optics. In the present paper we report the results of the investigation of the submicroscopic structure and nonlinear optical properties of (1-<i>x</i>)KNbO₃-<i>x</i>SiO₂ (KNS) glasses (x = 0.05-0.30) by XRD, SANS, electron microscopy and second harmonic generation (SHG) technique. Vitreous samples were fabricated by rapid melt cooling, via pressing the melt by steel plates, quenching between rotating metal rolls or splat cooling in air or nitrogen flow. Glasses with <i>x</i> < 0.15 are shown to possess a micro-inhomogeneous structure with regions enriched by SiO₂. On the contrary, as-quenched glasses with <i>x</i> > 0.15 are found by SANS to be homogeneous, but form nanostructures enriched by SiO₂ after heat-treatment. At temperatures below ~(<i>T</i>_<i>g</i> + 50 °C), SiO₂-enriched regions grow slightly, whereas their chemical composition shifts considerably closer to SiO₂. The data on the nano-inhomogeneous structure enables clarifying the complicated <i>T</i>_<i>g</i>(<i>x</i>) dependence of KNS glasses. SHG-active KNbO₃ phase precipitates at later stages of crystallization when the glass starts to lose its transparency, and crystallization of perovskite-like KNbO₃ is accompanied by the enhancement of SHG efficiency by several orders of magnitude.Read less <