A search for a Higgs boson decaying into a pair of electrons or muons and a photon is described. Higgs boson decays to a Z boson and a photon ($\mathrm{H}\to\mathrm{Z}\gamma\to\ell\ell\gamma$, $\ell=\mathrm{e}$ or $\mu$), or to two photons, one of which has an internal conversion into a muon pair ($\mathrm{H}\to\gamma^{*}\gamma\to\mu\mu\gamma$) were considered. The analysis is performed using a data set recorded by the CMS experiment at the LHC from proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. No significant excess above the background prediction has been found. Limits are set on the cross section for a standard model Higgs boson decaying to opposite-sign electron or muon pairs and a photon. The observed limits on cross section times the corresponding branching fractions vary between 1.4 and 4.0 (6.1 and 11.4) times the standard model cross section for $\mathrm{H}\to\gamma^{*}\gamma\to\mu\mu\gamma$ ($\mathrm{H}\to\mathrm{Z}\gamma\to\ell\ell\gamma$) in the 120-130 GeV mass range of the $\ell\ell\gamma$ system. The $\mathrm{H}\to\gamma^*\gamma\to\mu\mu\gamma$ and $\mathrm{H}\to\mathrm{Z}\gamma\to\ell\ell\gamma$ analyses are combined for $m_\mathrm{H} =$ 125 GeV, obtaining an observed (expected) 95% confidence level upper limit of 3.9 (2.0) times the standard model cross section.

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