Cesium (Cs) contamination has become an increasing environmental concern because of its high mobility, persistence, and ecological risks. Mosses have attracted considerable attention as bioindicators owing to their high environmental sensitivity; however, their responses to Cs remain poorly understood. In this study, the moss Hypnum plumaeforme was exposed to Cs⁺ (5, 50 and 500 mg L⁻¹) stress, and its morphological characteristics, antioxidant responses, photosynthetic performance and metabolomic alterations were comprehensively investigated. Low Cs⁺ exposure induced early oxidative signaling by increased H₂O₂ accumulation and slight stimulation of photosynthesis, whereas moderate stress triggered pronounced superoxide production and activation of antioxidant defenses. High Cs⁺ exposure caused severe cellular deformation, disruption of photosystem II, excessive hydroxyl radical accumulation, and collapse of the coordinated antioxidant–osmotic regulatory network. Untargeted LC–MS metabolomics further revealed substantial metabolic reprogramming under severe stress, including inhibition of carbon metabolism, significant enrichment of tryptophan metabolism, and accumulation of defensive secondary metabolites, indicating a shift in metabolic resources from growth toward stress defense. Collectively, these findings demonstrate the physiological responses of H. plumaeforme to Cs⁺ in a concentration-dependent manner. The high sensitivity of oxidative biomarkers, chlorophyll fluorescence parameters and metabolic signatures highlights the potential of H. plumaeforme as a bioindicator for assessment of cesium contamination.