  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
$\tilde{t}_1 \rightarrow t^{(*)} \tilde\chi^0_1$ and $\tilde{t}_1\rightarrow
b\tilde\chi^\pm_1 \rightarrow b W^{(*)} \tilde\chi^0_1$, where $\tilde\chi^0_1$
($\chi^\pm_1$) 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 $t \tilde\chi^0_1$, top-squark masses in the range 450-950 GeV
are excluded for $\tilde\chi^0_1$ masses below 160 GeV. In the case where
$m_{\tilde{t}_1}\sim m_t+m_{\tilde\chi^0_1}$, top-squark masses in the range
235-590 GeV are excluded.
