Abnormalities in the JAK/STAT signaling pathway lead to many diseases such as immunodeficiency, inflammation, and cancer. Herein, we designed and synthesized a series of 4-amino-(1<i>H</i>)-pyrazole derivatives as potent JAKs inhibitors for cancer treatment. Results from <i>in vitro</i> protein kinase inhibition experiments indicated that compounds <b>3a</b>-<b>f</b> and <b>11b</b> are potent JAKs inhibitors. For example, the IC<sub>50</sub> values of compound <b>3f</b> against JAK1, JAK2, and JAK3 were 3.4, 2.2, and 3.5 nM, respectively. In cell culture experiments, compound <b>3f</b> showed potent antiproliferative activity against various cell lines (PC-3, HEL, K562, MCF-7, and MOLT4) at low micromolar levels, while compound <b>11b</b> showed selective cytotoxicity at submicromolar levels against HEL (IC<sub>50</sub>: 0.35 μM) and K562 (IC<sub>50</sub>: 0.37 μM) cell lines. It is worth noting that both <b>3f</b> and <b>11b</b> showed more potent antiproliferative activities than the approved JAKs inhibitor Ruxolitinib.