Fifty-six Dunlins (Calidris alpina) that died accidentally at banding sites in Buck Bay (southern Baltic Sea), Poland, between 1998–2001 were sexed by dissection and measured. Measurements and sexes of these 32 females and 24 males were used to derive a discriminate function to help predict sexes of the C. a. alpina race using phenotypic measurements (bill, tarsus, and wing). Another sample of 16 female and 19 male Dunlins was used to validate this function. The best discriminate function for predicting the sex of juvenile Dunlins in the Southern Baltic included bill, tarsus, and wing length. Among single measurements, wing length was the poorest predictor of sex. Bill length was the best indicator, correctly identifying 91% of juvenile Dunlins in the validation sample. Longer-billed females were more likely to be mis-classified than shorter-billed males. The bills of juvenile Dunlins migrating through the Southern Baltic region in the autumn are still growing, and it appears as if these long-billed juvenile female Dunlins take more time to reach their final bill size than males.