Entangled photon pairs from spontaneous parametric down-conversion (SPDC)1 are central to many quantum applications2-6. SPDC is typically performed in non-centrosymmetric systems7 with an inherent second-order nonlinearity (χ(2))8-10. We demonstrate strong narrowband SPDC with an on-chip rate of 0.8 million pairs per second in Si3N4. Si3N4 is the pre-eminent material for photonic integration and also exhibits the lowest waveguide loss (which is essential for integrated quantum circuits). However, being amorphous, silicon nitride lacks an intrinsic χ(2), which limits its role in photonic quantum devices. We enabled SPDC in Si3N4 by combining strong light-field enhancement inside a high optical Q-factor microcavity with an optically induced space-charge field. We present narrowband photon pairs with a high spectral brightness. The quantum nature of the down-converted photon pairs is verified through coincidence measurements. This light source, based on Si3N4 integrated photonics technology, unlocks new avenues for quantum systems on a chip.