A search for pair production of a scalar partner of the top quark in events with four or more jets plus missing transverse momentum is presented. An analysis of 36.1 fb−1 of s=13 TeV proton-proton collisions collected using the ATLAS detector at the LHC yields no significant excess over the expected Standard Model background. To interpret the results a simplified supersymmetric model is used where the top squark is assumed to decay via t˜1→t(∗)χ˜10 and t˜1→bχ˜1±→bW(∗)χ˜10, where χ1 0(χ1 ±) denotes the lightest neutralino (chargino). Exclusion limits are placed in terms of the top-squark and neutralino masses. Assuming a branching ratio of 100% to tχ˜10, top-squark masses in the range 450–1000 GeV are excluded for χ˜10 masses below 160 GeV. In the case where mt˜1∼mt+mχ˜10, top-squark masses in the range 235–590 GeV are excluded.