Several studies indicate that spindle microtubules determine the position of the cleavage plane at the end of cell division, but their exact role in triggering the formation and ingression of the cleavage furrow is still unclear. Here we show that in Drosophila depletion of either the GAP (GTPase-activating protein) or the kinesin-like subunit of the evolutionary conserved centralspindlin complex prevents furrowing without affecting the association of astral microtubules with the cell cortex. Moreover, time-lapse imaging indicates that astral microtubules serve to deliver the centralspindlin complex to the equatorial cortex just before furrow formation. However, when the GAP-signaling component was mislocalized around the entire cortex using a membrane-tethering motif, this caused ectopic furrowing even in the absence of its motor partner. Thus, the GAP component of centralspindlin is both necessary and sufficient for furrow formation and ingression and astral microtubules provide a route for its delivery to the cleavage site.