The present disclosure relates to a diagnostic marker for breast cancer, having thioredoxin-1 as an active ingredient, and to a diagnostic kit for breast cancer using the same. The thioredoxin-1 is overexpressed in human breast cancer tissue so as to enable the early diagnosis of breast cancer or the early prediction prognosis of breast cancer, and therefore has a valuable use as a diagnostic marker for breast cancer. The present disclosure further relates to a method for the diagnosis of breast cancer comprising measuring serum thioredoxin 1 level. In addition, the method is useful in the early diagnosis of breast cancer thanks to its high diagnostic sensitivity and selectivity.