This article reports a study on a turbulence-free Young's double-slit interferometer. When the environmental turbulence blurs out the classic Young's double-slit interference completely, a two-photon interference pattern is still observable from the measurement of intensity or photon number fluctuation correlation. This two-photon interferometer always produces a turbulence-free interference pattern, when the double-slit interferometer is utilizing both first-order spatially incoherent light and spatially coherent light. This type of two-photon interferometer establishes new capabilities in optical observations and sensing measurements that require high sensitivity and stability. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement