  This letter reports the observation of $W(\ell\nu)\gamma\gamma$ production in
proton-proton collisions. This measurement uses the full Run 2 sample of events
recorded at a center-of-mass energy of $\sqrt{s} = 13$ TeV by the ATLAS
detector at the LHC, corresponding to an integrated luminosity of 140
fb$^{-1}$. Events with a leptonically-decaying $W$ boson and at least two
photons are considered. The background-only hypothesis is rejected with an
observed and expected significance of $5.6$ standard deviations. The inclusive
fiducial production cross section of $W(e\nu)\gamma\gamma$ and
$W(\mu\nu)\gamma\gamma$ events is measured to be $\sigma_{\mathrm{fid}} = 13.8
\pm 1.1 (\mathrm{stat}) \substack{+2.1 \\ -2.0} (\mathrm{syst}) \pm 0.1
(\mathrm{lumi})$ fb, in agreement with the Standard Model prediction.
