Abstract Hematological diseases, including leukemia, lymphoma, and multiple myeloma, are characterized by high heterogeneity with diverse cellular subpopulations. Single‐cell RNA sequencing (scRNA‐seq), a transformational technology, provides deep insights into cell‐to‐cell variation in tumor and microenvironment, allows high‐resolution dissection of the pathogenic mechanisms of diseases, and affords potential clinical utilities. Recent developments in single‐cell transcriptomics and associated technologies and their applications in hematological disorders for unraveling cellular subpopulations, disease pathogenesis, patient stratification, and therapeutic responses are summarized.