Allelopathy, the chemical interaction of plants by their secondary metabolites with surrounding organisms, profoundly influences their functional features. Lichens, symbiotic associations of fungi and algae and/or cyanobacteria, produce diverse secondary metabolites, among other usnic acid, which express to have potent biological activities. Mosses, i.e. Physcomitrium patens, share the habitat with other organisms including lichens, experiencing the allelopathic effects of their metabolites. In this study, we investigated the interference of usnic acid on P. patens as inferred by proteomics, shedding light on the physiological response of this moss. Our results revealed spreading inhibition of of P. patens, under usnic acid treatment (reduction of protonemal patches and enhanced gametophore growth), along with significant alterations in the moss proteome. The results showed that structural proteins and those involved in vital life function are stable or even increased under the treatments. Thus, proteins associated with photosynthesis, stress response, and defense mechanisms were up-regulated, while those involved in energy metabolism and protein biosynthesis were down-regulated. These findings enhance our understanding of moss responses to allelopathic stress and lay the groundwork for future investigations into the functional significance of specific proteins in moss adaptation to environmental challenges.