Stable isotopes are commonly used to characterize food web structure and resource use by aquatic organisms. White muscle is generally preferred for stable isotope analysis of fishes. However, obtaining white muscle tissue typically requires lethal take or invasive sampling techniques, which are undesirable for small-bodied species or those of conservation concern. We assessed the use of fish fin as a nonlethal alternative to muscle tissue for stable isotope analysis of four small-bodied fishes native to the upper Red River drainage of Texas and Oklahoma, USA: plains minnow Hybognathus placitus, prairie chub Macrhybopsis australis, Red River shiner Alburnops bairdi, and Red River pupfish Cyprinodon rubrofluviatilis. Fin isotope values were strong predictors of both δ¹⁵N and δ¹³C muscle isotope values (ANCOVA: δ¹⁵N: F_1,451 = 5312.09, P < 0.001; δ¹³C: F_1,451 = 7864.39, P < 0.001), although isotopic composition varied among species for δ¹³C (F_3,451 = 4.29, P < 0.01). Species-specific regression models indicated positive linear relationships between fin and muscle isotope values (δ¹⁵N: P < 0.001, R² ≥ 0.80; δ¹³C: P < 0.001, R² ≥ 0.83) that did not vary significantly with body size or age of individuals. We suggest minimum total length thresholds for least destructive fin clipping at 70 mm, 62 mm, and 48 mm for plains minnow, prairie chub, and Red River pupfish, respectively. Nonlethal fin clipping may not be viable for Red River shiner within the size range reported here as multiple fins were required for routine analysis. Overall, we conclude that fin tissue may be used for δ¹⁵N and δ¹³C assessments to mitigate lethal take of imperiled, small-bodied fishes.