Abstract Birch syrup is a forest product produced from tree sap using methods similar to those used in maple syrup production. This study presents the first measurements of the carbon isotope ratio of birch syrup, which is ultimately a function of tree physiology and environmental variables. Birch syrup has an average δ13C value = −25.31 ± 0.62‰ VPDB (1σ, n = 28), which is comparable to maple syrup. A few other tree syrups were analyzed for comparison: black walnut syrup has an average δ13C value = −25.52 ± 0.33‰ (n = 4) and one butternut syrup has a δ13C value = −23.88‰. All of these syrups are within the range of carbon isotope ratios typical of C3 plants, and are distinct from the carbon ratios of common C4 sugars derived from corn and sugarcane (δ13C value≈ −11 to −12‰). Detection of exogenous C4 sugars using carbon isotope ratio can be applied to birch syrup in the same way as it is used for other C3 foods such as maple syrup, honey, and fruit products.