Recent ATLAS searches for rare and lepton-flavour-violating (LFV) Higgs-boson decays are presented. Rare Standard Model decays, including \(H\to\mu\mu\), \(H\to Z\gamma\), and \(H\to\gamma^*\gamma\to\ell\ell\gamma\), provide stringent tests of the Higgs sector and sensitive probes of possible new physics. Searches for the LFV decays \(H\to e\tau\) and \(H\to\mu\tau\), which are absent in the Standard Model at experimentally accessible rates, provide complementary sensitivity to physics beyond the Standard Model. Results based on LHC Run 2 and Run 3 data, including combinations of the two datasets, are presented.