Phytophagous mites of the superfamily Eriophyoidea are capable of inducing gall formation on various organs of higher vascular plants. However, the question of how gallogenesis affects leaf assimilation surface and photosynthetic activity of the host plants is poorly understood. We have examined the influence of gall-forming mites from the genera Eriophyes and Acalitus on leaf size, shape and photosynthetic pigment content in five deciduous tree species near the city of Tyumen, West Siberia. The gall mite infestation resulted in chlorosis, destruction of photosynthetic apparatus in gall-infected leaf parts, leaf deformation and a decrease in leaf area. The magnitude of the effects on leaf size and shape varied among the studied mite–tree systems and did not depend on the infection severity. On the contrary, chlorophyll and carotenoid amounts per leaf decreased in an infection severity-dependent manner in all mite–host plant variants. Mite-induced galls did not influence the pigment concentration in green uninfected gaps between galls. Additionally, the chlorophyll amount in the infected leaves has decreased due to the destruction of the pigment complex in the galled leaf areas and a decrease of the whole-leaf area. As a result, the losses of chlorophylls and carotenoids in leaves of all studied trees were directly related to the infection severity (quantified as the proportion of damaged leaf area to the total leaf area). Our results may help developing an approach to assess the effect of gall mites on the chlorophyll content and the photosynthetic productivity of trees, based on the direct or remote analysis of damaged leaf surface.