Stable Sn isotope ratios are emerging as a novel tracer for a wide range of geological processes; however, the Sn isotopic baseline of the upper continental crust (UCC) is not yet well-constrained. Here, we report high-precision Sn isotope data of a wide range of UCC samples, including granites, pegmatites, and sediments, to document the Sn isotopic composition of UCC. Significant variations in δ122/118Sn3161a (per mil deviation in 122Sn/118Sn relative to NIST 3161a) values are revealed for I-type (δ122/118Sn3161a = 0.025 ± 0.026‰ to 0.495 ± 0.046‰, n = 20) and S-type (δ122/118Sn3161a = 0.156 ± 0.018‰ to 0.501 ± 0.075‰, n = 22) granites. More extreme Sn isotope variability is observed for pegmatites, which have δ122/118Sn3161a from 0.256 ± 0.047‰ to 0.930 ± 0.049‰ (n = 13). The δ122/118Sn3161a of I-type granites decrease with decreasing TFe2O3 (total Fe as Fe2O3) and MgO contents and are attributed to the segregation of Fe-bearing minerals. Conversely, the Sn isotope variation of S-type granites appears to reflect source heterogeneity, whereas the Sn isotope variability of pegmatites may reflect fluid activity. In contrast, the loess samples display homogeneous δ122/118Sn3161a values (0.132 ± 0.034‰ to 0.239 ± 0.020‰, n = 20) that show no correlation with the degree of chemical weathering, suggesting that loess is representative of the average Sn isotope composition of UCC. The δ122/118Sn3161a values of modern sediments and sedimentary rocks range from 0.080‰ to 0.490‰ (n = 25) and the Sn isotope variations may be related to chemical weathering or sediment provenance. Based on the lithology-weighted average δ122/118Sn3161a of UCC samples (41 granites and 45 sediments) in this study, the δ122/118Sn3161a value of UCC is estimated to be 0.233 ± 0.099‰, providing a reference point for further investigations. Tin is isotopically lighter in UCC compared to the mantle, owing to magmatic differentiation and crustal evolution processes.