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An unidentified $\gamma$-ray source 4FGL J1838.2+3223 has been proposed as apulsar candidate. We present optical time-series multi-band photometry of itslikely optical companion obtained with the 2.1-m telescope of ObservatorioAstron\'omico Nacional San Pedro M\'artir, Mexico. The observations and thedata from the Zwicky Transient Facility revealed the source brightnessvariability with a period of $\approx$4.02 h likely associated with the orbitalmotion of the binary system. The folded light curves have a single sine-likepeak per period with an amplitude of about three magnitude accompanied by fastsporadic flares up to one magnitude level. We reproduce them modelling thecompanion heating by the pulsar. As a result, the companion side facing thepulsar is strongly heated up to 11300$\pm$400 K, while the temperature of itsback side is only 2300$\pm$700 K. It has a mass of 0.10$\pm$0.05 ${\rmM}_\odot$ and underfills its Roche lobe with a filling factor of$0.60^{+0.10}_{-0.06}$. This implies that 4FGL J1838.2+3223 likely belongs tothe `spider' pulsar family. The estimated distance of $\approx$3.1 kpc iscompatible with Gaia results. We detect a flare from the source in X-rays andultraviolet using Swift archival data and another one in X-rays with theeROSITA all-sky survey. Both flares have X-ray luminosity of $\sim$10$^{34}$erg s$^{-1}$ which is two orders of magnitude higher than the upper limit inquiescence obtained from eROSITA assuming spectral shape typical for spiderpulsars. If the spider interpretation is correct, these flares are among thestrongest flares observed from non-accreting spider pulsars.

Nature of 4FGL J1838.2+3223: a flaring `spider' pulsar candidate