The general consensus is that immune cells are exposed to physiological hypoxia in vivo (PhyO2, 2-5% PO2). However, functional studies of B cells in hypoxic conditions are sparse. Recently, we reported the expression in mouse B cells of TASK-2, a member of pH-sensitive two-pore domain K+ channels with background activity. In this study, we investigated the response of K+ channels to sustained PhyO2 (sustained hypoxia [SH], 3% PO2 for 24 h) in WEHI-231 mouse B cells. SH induced voltage-independent background K+ conductance (SH-Kbg) and hyperpolarized the membrane potential. The pH sensitivity and the single-channel conductance of SH-Kbg were consistent with those of TASK-2. Immunoblotting assay results showed that SH significantly increased plasma membrane expressions of TASK-2. Conversely, SH failed to induce any current following small interfering (si)TASK-2 transfection. Similar hypoxic upregulation of TASK-2 was also observed in splenic primary B cells. Mechanistically, upregulation of TASK-2 by SH was prevented by si hypoxia-inducible factor-1a (HIF-1a) transfection or by YC-1, a pharmacological HIF-1a inhibitor. In addition, TASK-2 current was increased in WEHI-231 cells overexpressed with O2-resistant HIF-1a. Importantly, [Ca2+]c increment in response to BCR stimulation was significantly higher in SH-exposed B cells, which was abolished by high K+-induced depolarization or by siTASK-2 transfection. The data demonstrate that TASK-2 is upregulated under hypoxia via HIF-1α-dependent manner in B cells. This is functionally important in maintaining the negative membrane potential and providing electrical driving force to control Ca2+ influx.