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 5122/118Sn3161a (per mil deviation in 122Sn/118Sn relative to NIST 3161a) values are revealed for I-type (5122/118Sn3161a = 0.025 f 0.026%o to 0.495 f 0.046%o, n = 20) and S-type (5122/118Sn3161a = 0.156 f 0.018%o to 0.501 f 0.075%o, n = 22) granites. More extreme Sn isotope variability is observed for pegmatites, which have 5122/118Sn3161a from 0.256 f 0.047%o to 0.930 f 0.049%o (n = 13). The 5122/118Sn3161a 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 5122/118Sn3161a values (0.132 f 0.034%o to 0.239 f 0.020%o, 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 5122/118Sn3161a values of modern sediments and sedimentary rocks range from 0.080%o to 0.490%o (n = 25) and the Sn isotope variations may be related to chemical weathering or sediment provenance. Based on the lithology-weighted average 5122/118Sn3161a of UCC samples (41 granites and 45 sediments) in this study, the 5122/118Sn3161a value of UCC is estimated to be 0.233 f 0.099%o, 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.