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The realization of ultrahigh quality upconverted single‐mode lasing in wavelength‐scale cavity is still a great challenge, because of limitation of microcavity quality, gain medium, and low spontaneous emission coupling efficiency β. In this work, it is demonstrated that inorganic cesium lead bromide (CsPbBr3) perovskite spheres with size of ≈780 nm serve as a gain medium and whispering‐gallery cavity simultaneously for two‐photon excited single‐mode laser. Lasing mode with a very narrow linewidth (≈0.037 nm) is realized, corresponding to an ultrahigh Q factor ≈1.5 × 104. Surprisingly, the lasing performance of such small microcavity is best among reported naturally formed semiconductor microcavities. Furthermore, room‐temperature single‐mode lasing at same wavelength can be obtained at any angle between −30° and 30°, showing the nearly uniform emission output of the spherical microcavity. This work paves a simple and straight way to obtain ultrahigh Q two‐photon excited single‐mode lasing in micro/nanostructures, holding great promise for practical applications in frequency upconversion, high sensitive sensing in nano‐optics. Ultrahigh quality upconverted single‐mode lasers in individual all‐inorganic CsPbBr3 sub‐micrometer spherical whispering‐gallery mode microcavities are realized. The upconverted single‐mode laser exhibits ultrahigh Q factor (≈1.5 × 104) and very small linewidth (≈0.037 nm) in such a sub‐micrometer spherical (D < 1 µm) cavity, which is the best in all the reported natural nano/microcavity lasers. [ABSTRACT FROM AUTHOR]