OBJECTIVE Physical activity (PA) is important for recovery after a breast cancer diagnosis; however, women's motivation to engage in PA can be impacted by disease and/or treatment, and can therefore be a challenge. This study explored factors associated with PA levels during chemotherapy among women with breast cancer. METHODS The study had a cross-sectional descriptive and comparative design using a study-specific questionnaire. One hundred women with breast cancer receiving adjuvant chemotherapy were included. Data were analysed by Pearson's correlation coefficient and linear regression. The open question was subjected to manifest content analysis. RESULTS Identified factors associated with engaging in PA during chemotherapy treatment were: being physically active before diagnosis, and the information given by the oncology nurse before the treatment start. The physically active women experienced higher psychological wellbeing, less fatigue, and faster recovery after treatment. They also experienced an overall feeling of fitness. CONCLUSIONS It seems that PA is associated with less fatigue, better recovery between chemotherapy treatments, and a better mental condition leading to wellbeing. Information given by the oncology nurse may be an important factor for being physically active. Women with breast cancer need to get specific advice about and support in engaging in PA to feel better during chemotherapy treatment. Further research is required to develop guidelines for advice about and support regarding PA during chemotherapy treatment.