Purpose <p>Oncofertility support plays a critical role in coping with fertility issues among women with breast cancer. There is a growing demand for reliable tools to assess and enhance oncofertility support. This study aimed to validate a tool assessing oncofertility support.</p> Methods <p>The study involved two phases. First, insights from healthcare professionals and women with breast cancer were used to modify the Fertility Information Support Scale, resulting in a 22-item Oncofertility Support Scale. Second, a cross-sectional study was conducted among 343 participants aged 18–40 to assess the scale’s content validity, structural validity, measurement invariance, convergent validity, known-group validity, and internal consistency reliability.</p> Results <p>Exploratory factor analysis yielded a 19-item scale across four dimensions: information support on fertility impact, information support on fertility preservation, fertility and sexual guidance, and fertility communication and supportive networks. Confirmatory factor analysis confirmed the scale’s structure. Multi-group confirmatory factor analysis showed measurement invariance across age and fertility desire groups. The content validity for the scale was 0.95. Oncofertility support showed a weak association with medical social support. Average Variance Extracted (AVE ≥ 0.63) and Composite Reliability (CR ≥ 0.90) supported the convergent validity of the scale. Women aged ≤ 30 reported higher information support on fertility preservation and fertility communication and supportive networks than those older than 30. Both Cronbach’s alpha and McDonald’s ω were 0.95.</p> Conclusion <p>The Oncofertility Support Scale is a valid and reliable measure for assessing oncofertility support among women with breast cancer.</p>