A search for direct pair production of scalar partners of the top quark (top squarks or scalar third-generation up-type leptoquarks) in the all-hadronic 𝑡𝑡¯ plus missing transverse momentum final state is presented. The analysis of 139 fb−1 of 𝑠√=13 TeV proton–proton collision data collected using the ATLAS detector at the LHC yields no significant excess over the Standard Model background expectation. To interpret the results, a supersymmetric model is used where the top squark decays via 𝑡̃ →𝑡(∗)𝜒̃ 01, with 𝑡(∗) denoting an on-shell (off-shell) top quark and 𝜒̃ 01 the lightest neutralino. Three specific event selections are optimised for the following scenarios. In the scenario where 𝑚𝑡̃ >𝑚𝑡+𝑚𝜒̃ 01, top squark masses are excluded in the range 400–1250 GeV for 𝜒̃ 01 masses below 200 GeV at 95% confidence level. In the situation where 𝑚𝑡̃ ∼𝑚𝑡+𝑚𝜒̃ 01, top squark masses in the range 300–630 GeV are excluded, while in the case where 𝑚𝑡̃ <𝑚𝑊+𝑚𝑏+𝑚𝜒̃ 01 (with 𝑚𝑡̃ −𝑚𝜒̃ 01≥5 GeV), considered for the first time in an ATLAS all-hadronic search, top squark masses in the range 300–660 GeV are excluded. Limits are also set for scalar third-generation up-type leptoquarks, excluding leptoquarks with masses below 1240 GeV when considering only leptoquark decays into a top quark and a neutrino.