Purpose: Increased sedentary behavior during cancer treatment is common, which may have negative long-term health effects. Understanding patients' experience of sedentary behavior during neo- or adjuvant cancer treatment may be crucial in developing effective support for patients to reduce sedentary behavior. Therefore, the present study aimed to explore sedentary behavior in patients undergoing neo- or adjuvant cancer treatment. Methods: Eleven interviews were conducted with patients undergoing treatment for breast, prostate, and colorectal cancer. Participants were recruited from a university hospital in Sweden. Interviews were analyzed phenomenologically, and the results were presented as descriptions of the phenomenon sedentary behavior. Results: The overarching theme of sedentary behavior during cancer treatment was that sedentary behavior is experienced through being physically active or not. Furthermore, experiences of sedentary behavior increased with side effects and varied depending on the type and phase of cancer treatment, meaning that sedentary behavior is an adjustment to side effects. Additionally, sedentary behavior was influenced by life circumstances and social interaction, such as work status and having social support. Finally, sedentary behavior is influenced by strategies and motivations, such as the perceived benefits of physical activity and self-image. Conclusions: Sedentary behavior is difficult for patients to discern, which is why health care personnel may need to help patients by increasing awareness of the negative impact of sedentary behavior in a way that does not stigmatize patients. Furthermore, developing support that targets periods with more side effects and helping patients reduce sedentary behavior throughout changing life circumstances may be helpful.