Janus kinase 3 (JAK3) is a potential target for the treatment of hematological malignancies. Herein, we report the discovery of a series of new orally bioavailable irreversible JAK3 kinase inhibitors. The representative compound 12n potently inhibited JAK3 kinase activity with an IC50 value of 1.2 nM and was more than 900-fold selective over JAK1, JAK2, and Tyk2. Cell-based assays revealed that 12n significantly suppressed phosphorylation of JAK3 and the downstream effectors STAT3/5 and also robustly restrained proliferation of BaF3 cells transfected with JAK3M511I activating mutation and human leukemia U937 cells harboring JAK3M511I with IC50 values of 22.9 and 20.2 nM, respectively. More importantly, 12n showed reasonable pharmacokinetic (PK) properties, and oral administration of 12n at a dose of 50 mg/kg twice daily led to tumor regression in a U937 cell inoculated xenograft mouse model. Thus, 12n represents a promising lead compound for further optimization to discover new therapeutic agents for hematological malignancies.