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 ˜ 1 0 $$ {\tilde{t}}_1\to {t}^{\left(\ast \right)}{\tilde{\upchi}}_1^0 $$ and t ˜ 1 → b χ ˜ 1 ± → b W ∗ χ ˜ 1 0 $$ {\tilde{t}}_1\to b{\tilde{\upchi}}_1^{\pm}\to b{W}^{\left(\ast \right)}{\tilde{\upchi}}_1^0 $$ , 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 t χ ˜ 1 0 $$ t{\tilde{\upchi}}_1^0 $$ , top-squark masses in the range 450–1000 GeV are excluded for χ ˜ 1 0 $$ {\tilde{\upchi}}_1^0 $$ masses below 160 GeV. In the case where m t ˜ 1 ∼ m t + m χ ˜ 1 0 $$ {m}_{{\tilde{t}}_1}\sim {m}_t+{m}_{{\tilde{\chi}}_1^0} $$ , top-squark masses in the range 235–590 GeV are excluded.