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 $ sqrt{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 $ {tilde{t}}_1to {t}^{left(ast right)}{tilde{\upchi}}_1^0 $ and $ {\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{tilde{upchi}}_1^0 $ , top-squark masses in the range 450?1000 GeV are excluded for $ {tilde{upchi}}_1^0 $ masses below 160 GeV. In the case where $ {m}_{{tilde{t}}_1}sim {m}_t+{m}_{{tilde{chi}}_1^0} $ , top-squark masses in the range 235?590 GeV are
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