These proceedings present recent measurements of rare multiboson processes performed by the
ATLAS Collaboration at the LHC using proton--proton collision data collected at
$\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of 140 fb$^{-1}$.
The results include the observation of $W^{+}W^{-}\gamma$ production ($5.9\sigma$),
the first evidence for $ZZ\gamma$ production ($4.4\sigma$), the observation of electroweak
$\gamma\gamma jj$ production ($6.2\sigma$), and the observation of electroweak diboson production
via vector-boson scattering in semileptonic final states ($7.4\sigma$). Fiducial cross-sections
are measured for each process and found to be compatible with Standard Model predictions, and these
measurements are used to constrain anomalous quartic gauge-boson couplings within the framework of
Effective Field Theory.