Atmospheric carbon dioxide is the raw material for the biosphere. Therefore, changes in the carbon isotopic composition of the atmosphere will influence the terrestrial δ¹³C signals we interpret. However, reconstructing the atmospheric δ¹³C value in the geologic past has proven challenging. Land plants sample the isotopic composition of CO₂ during photosynthesis. We use a model of carbon isotopic fractionation during C3 photosynthesis, in combination with a meta–data set (519 measurements from 176 species), to show that the δ¹³C value of atmospheric CO₂ can be reconstructed from the isotopic composition of plant tissue. Over a range of pCO₂ (198–1300 ppmv), the δ¹³C value of plant tissue does not vary systematically with atmospheric carbon dioxide concentration. However, environmental factors, such as water stress, can influence the δ¹³C value of leaf tissue. These factors explained a relatively small portion of variation in the δ¹³C value of plant tissue in our data set and emerged strongly only when the carbon isotopic composition of the atmosphere was held constant. Members of the Poaceae differed in average δ¹³C value, but we observed no other differences correlated with plant life form (herbs, trees, shrubs). In contrast, over 90% of the variation the carbon isotopic composition of plant tissue was explained by variation in the δ¹³C value of the atmosphere under which it was fixed. We use a subset of our data spanning a geologically reasonable range of atmospheric δ¹³C values (−6.4‰ to −9.6‰) and excluding C3 Poaceae to develop an equation to reconstruct the δ¹³C value of atmospheric CO₂ based on plant values. Reconstructing the δ¹³C value of atmospheric CO₂ in geologic time will facilitate chemostratigraphic correlation in terrestrial sediments, calibrate pCO₂ reconstructions based on soil carbonates, and offer a window into the physiology of ancient plants.