Searches for new resonances that decay either to pairs of top quarks or a top and a $b$-quark are presented. The searches are performed with the ATLAS experiment at the LHC using $3.2$~fb$^{-1}$ proton-proton collision data at $\sqrt{s}=13$~TeV and $20.3$~fb$^{-1}$ of $8$~TeV collision data. The invariant mass spectrum of hypothetical resonances are examined for local excesses or deficits that are inconsistent with the Standard Model prediction. No significant deviation from the prediction is found so far.