In lichens, ribitol is known as a carbon storage compound, an osmotic agens involved effectively in cell compartments protection during dehydration of lichen thalli and as a cryoprotective compound. In our study, we investigated the effect of ribitol on photochemical processes of photosynthesis in foliose lichens [Lasallia pustulata (L.) Mérat., Umbilicaria hirsuta (Sw. ex Westr.) Hoffm.] at low temperature. The effects of three concentrations of ribitol, added externally to thalli segments on several chlorophyll (Chl) fluorescence parameters, were evaluated. The 72 h exposition to 8, 16, and 26 mM ribitol led to a concentration-dependent increase in FV/FM, decrease in non-photochemical quenching (NPQ) but no change in quantum yield of photosystem II photochemistry (ΦPSII) values at −5 °C). At higher temperature (0, +5 °C), no effect of ribitol addition on the photosynthetic parameters was apparent.