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√=13s=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~01t~1→t(∗)χ~10 and t~1→bχ~±1→bW(∗)χ~01t~1→bχ~1±→bW(∗)χ~10, where χ 1 0 (χ 1 ± ) 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~01tχ~10, top-squark masses in the range 450–1000 GeV are excluded for χ~01χ~10 masses below 160 GeV. In the case where mt~1∼mt+mχ~01mt~1∼mt+mχ~10, top-squark masses in the range 235–590 GeV are excluded.