Abstract Background: Head and neck cancer (HNC) patients undergoing radiotherapy (RT) require proper patient immobilization, for which a thermoplastic mask is used. Studies have reported a variety of patient experiences in using this device. Understanding these experiences in our local context is important to improve the current practice in the RT department. This study aimed at exploring the experiences of HNC patients using thermoplastic masks during RT at Ocean Road Cancer Institute in Dar es Salaam, Tanzania. Methods: In-depth interviews were conducted in Swahili for thirteen purposively selected HNC patients who were undergoing RT treatment, using a semi-structured interview guide. Data were transcribed verbatim and translated into English and thematic analysis was done. Results: There was a diverse understanding about thermoplastic masks among patients, and experiences of thermoplastic mask wearing varied, with patients expressing an experience of anxiety and discomfort during the first days of treatment and pain at later days of treatment. Conclusion: Enhancing communication and ensuring the availability of staff numbers required during thermoplastic mask construction and treatment improves patients' understanding and comfortability as well as reduces anxiety. Also, regular assessment of pain and proper management of radiation-induced side effects among HNC patients is important during RT treatment.