Preparation of high-quality two-dimensional (2D) transition metal dichalcogenides (TMDCs) is the precondition for realizing their applications. However, the synthesized 2D TMDCs (e.g., MoS2) crystals suffer from low quality due to the massive defects formed during the growth. Here, we report single-atom sulfur (S1) as a highly reactive sulfur species to grow ultrahigh-quality monolayer MoS2. Derived from battery waste, sulfurized polyacrylonitrile (SPAN) is found to be exclusive and efficient in releasing S1. The monolayer MoS2 prepared by SPAN exhibits an ultralow defect density of ∼7 × 1012 cm-2 and the narrowest photoluminescence (PL) emission peak with full-width at half-maximum of ∼47.11 meV at room temperature. Moreover, the statistical resonance Raman and low-temperature PL results further verify the significantly lower defect density and higher optical quality of SPAN-grown MoS2 than those of the conventional S-powder-grown samples. This work provides an effective approach for preparing ultrahigh-quality 2D single crystals, facilitating their industrial applications.