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Herbivory may offset climate change-driven treeline expansion into the tundra, but the strength of this effect is rarely quantified. This study leverages a unique semi-natural experiment involving Malla Strict Nature Reserve in northernmost Finland, where the reindeer herding regime shifted from being nearly ungrazed for several decades to being heavily grazed for the past two decades. This is contrasted by low grazing pressure in the adjacent herding district in Norway, which is separated by the border fence preventing free reindeer movement between the two countries. We aimed to quantify the effects of reindeer browsing and grazing on mountain birch treeline position and structure on both sides. We measured seedling numbers and the allometry of trees, vegetation composition, nutrient concentrations in soils and birch leaves, and radial tree growth. We found higher numbers of seedlings and saplings in the area with lower reindeer density, indicating that the treeline may be responding to climatic forcing by expanding into the tundra. Contrastingly, we observed almost no recruitment and treeline expansion in the area with high reindeer density. Furthermore, while birch leaves showed signs of nitrogen enrichment under high reindeer density, we found no differences in soil chemical composition or birch tree growth rates. Our results suggest that the high density of reindeer in Malla Strict Nature Reserve keeps the treeline in a browsing trap, thereby preventing climate change-driven forest expansion. These results are highly relevant for land management decisions that aim to preserve mountain tundra.