A search for direct pair production of the supersymmetric partner of the top quark, decaying via a scalar tau to a nearly massless gravitino, has been performed using 20 fb $$^{-1}$$ of proton–proton collision data at $$\sqrt{s}=8~\mathrm{TeV}$$ . The data were collected by the ATLAS experiment at the LHC in 2012. Top squark candidates are searched for in events with either two hadronically decaying tau leptons, one hadronically decaying tau and one light lepton, or two light leptons. No significant excess over the Standard Model expectation is found. Exclusion limits at $$95~\%$$ confidence level are set as a function of the top squark and scalar tau masses. Depending on the scalar tau mass, ranging from the $$87~\mathrm{GeV}$$ LEP limit to the top squark mass, lower limits between 490 and $$650~\mathrm{GeV}$$ are placed on the top squark mass within the model considered.