Background/Objectives: Breast cancer survivors often face an elevated risk of developing co-morbid chronic diseases, which may be exacerbated by nutritional inequalities. This study aimed to comprehensively assess the associations between nutrition literacy, dietary quality, and the risk of co-morbidity in breast cancer survivors in China. Methods: A cross-sectional study was conducted among cancer survivors enrolled in the Shanghai Cancer Rehabilitation Club from March to July 2023. The multivariable models of logistic regression, Poisson regression, and mediation analysis were used to explore the relationship between nutrition literacy, dietary quality, and co-morbid chronic diseases among breast cancer survivors in China. Results: The mean age of the 1552 female breast cancer survivors was 64.5 ± 7.02 years. Nutrition literacy was found to be low, with 49.81% of participants scoring above the threshold for adequate nutrition literacy. The median (IQR) diet quality distance, low bound score, and high bound score were 67.0 (55.0, 81.0), −39.0 (−51.0, −28.0), and 25.0 (16.0, 36.0), respectively. Of the total female breast cancer survivors, 67.27% were reported to have at least one comorbid chronic disease. Hypertension (37.32%) was the most common co-morbid chronic disease. Each score increase of 10 in nutrition literacy (AOR 0.88, 95%CI 0.808–0.962) was associated with a significant reduction in co-morbid chronic disease risk among breast cancer survivors. Poisson regression analysis of the number of chronic diseases was conducted, and consistently, an association between higher nutrition literacy levels and fewer chronic diseases was observed (AOR 0.94, 95%CI 0.911–0.971). The indirect effect of nutrition literacy on comorbidity risk through dietary quality was not significant (indirect effect = 0.994, 95%CI = 0.980–1.008). Conclusions: Nutrition literacy is paramount in breast cancer survivors’ dietary quality and prognosis. Enhancing nutrition literacy may be a crucial strategy for improving dietary quality and mitigating the risk of comorbid chronic diseases in this vulnerable population.