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Quantum dots (QDs) often exhibit unique behaviors because the reduction in lateral size leads to stronger quantum confinement effects and a higher surface-to-volume ratio in comparison with larger two-dimensional nanosheets. However, the preparation of homogeneous QDs remains a longstanding challenge. This work reports the preparation of high-yield and ultrasmall tin disulfide (SnS2) QDs by combining top–down and bottom–up approaches. The as-prepared SnS2 QDs have a uniform lateral size of 3.17 ± 0.62 nm and a thicknesses 2.39 ± 0.88 nm. A series of self-powered photoelectrochemical-type photodetectors (PDs) utilizing the SnS2 QDs as photoelectrodes are also constructed. Taking advantage of the tunable bandgaps and high carrier mobility of the SnS2, our PDs achieve a high photocurrent density of 16.38 μA cm−2 and a photoresponsivity of 0.86 mA W−1, and good long-term cycling stability. More importantly, the device can display obvious photoresponse, even at zero bias voltage (max), and greater weak-light sensitivity than previously reported SnS2-based PDs. Density functional theory calculation and optical absorption were employed to reveal the working mechanism of the SnS2 QDs-based PDs. This study highlights the prospective applications of ultrasmall SnS2 QDs and provides a new route towards future design of QDs-based optoelectronic devices.