Somatic stem cells maintain tissue homeostasis by dynamically adjusting proliferation and differentiation in response to stress and metabolic cues. Here we identify Ca2+ signalling as a central regulator of intestinal stem cell (ISC) activity inDrosophila. We show that dietary L-glutamate stimulates ISC division and gut growth. The metabotropic glutamate receptor (mGluR) is required in ISCs for this response, and for an associated modulation of cytosolic Ca2+ oscillations that results in sustained high cytosolic Ca2+ concentrations. High cytosolic Ca2+ concentrations induce ISC proliferation by regulating Calcineurin and CREB-regulated transcriptional co-activator (Crtc). In response to a wide range of dietary and stress stimuli, ISCs reversibly transition between Ca2+ oscillation states that represent poised or activated modes of proliferation, respectively. We propose that the dynamic regulation of intracellular Ca2+ levels allows effective integration of diverse mitogenic signals in ISCs to adapt their proliferative activity to the needs of the tissue.