AbstractLanthanide (Ln3+)‐based ultraviolet B (UVB) microlasers are highly desirable for diagnostics and phototherapy. Despite their progress, the potential applications of UVB microlasers are strongly hindered by their low optical gain, weak light confinements, and poor device repeatability. Herein, a novel all‐in‐one approach to solve the above limitations and realize mass‐manufactural UVB microlasers is reported. The gain coefficient at 289 nm is improved from two aspects, i.e., the enhanced absorption via LiYbF4:Tm(1mol%)@LiYbF4@LiLuF4 core–shell–shell nanocrystals and the suppression of competitive ultraviolet emissions. Consequently, by spin‐coating the solution onto a patterned SiO2 substrate, high‐quality Ln3+‐based microdisks are formed by self‐assembly on each SiO2 pillar and UVB whispering‐gallery‐mode lasers are realized. The resulted lasing threshold is an order of magnitude smaller than the shortest deep‐ultraviolet emission at 310.5 nm. Importantly, the lasing wavelengths and mode numbers of UVB lasers are highly controllable and repeatable, making them suitable for mass production for the first time.