Tenuipalpus micheli is a phytophagous mite known to cause considerable physiological disruption in host plants through its feeding on epidermal tissues. Understanding plant responses such as phenolic accumulation and alterations in photosynthetic efficiency is essential for evaluating the extent of stress induced by mite infestation. The present study aimed to assess the infestation dynamics of T. micheli on Syzygium cumini and to evaluate its impact on phenolic content and photosynthetic efficiency using chlorophyll fluorescence parameters. Field studies were conducted in Kerala to monitor seasonal population fluctuations of T. micheliin relation to environmental factors such as temperature and rainfall. Infestation symptoms were recorded, and comparative analyses were performed between infested and uninfested leaves. Total phenol content was estimated using the Folin–Ciocalteu method. Photosynthetic efficiency was analysed through chlorophyll fluorescence parameters, including minimum fluorescence (F0), maximum fluorescence (Fm), variable fluorescence (Fv), Fv/Fm ratio, performance index (PI), and the area above the fluorescence curve (Kautsky effect), measured using a Handy PEA analyser. The study identified S. cumini as a new host record for T. micheli. The mite population exhibited marked seasonal variation, with peak abundance during February–May and minimal incidence during the monsoon season, indicating a positive correlation with temperature and a negative influence of rainfall. Infestation resulted in characteristic foliar symptoms such as chlorosis, silvery patches, and necrosis. A significant increase in total phenol content was observed in infested leaves (1.05±0.03 mg/g) compared to uninfested leaves (0.49±0.02 mg/g), representing an increase of over 100%, which indicates activation of plant defense mechanisms. Chlorophyll fluorescence analysis revealed a marked decline in photosynthetic efficiency in infested leaves, with reduced Fv/Fm ratio and performance index, reflecting impairment of photosystem II activity. Additionally, reductions in fluorescence parameters and the Kautsky curve area suggest disruption of electron transport processes. Overall, T. micheliinfestation induces substantial biochemical and photochemical stress in S. cumini, affecting plant vitality and highlighting its potential as an emerging pest requiring monitoring and management.