BACKGROUND: Cancer is one of the major health problems across the world that poses a great threat to human health and active life in all age groups, causing numerous personal, familial, and social damages in physical, mental and spiritual dimensions. The aim of the present study was to investigate the effect of positive psychotherapy on happiness, character strengths, pleasure, commitment, and semantic measures in the lives of people with cancer. MATERIALS AND METHODS: Within the framework of a quasi-experimental design, 58 patients suffering from cancer meeting the inclusion criteria (disease history and married) were selected via purposeful sampling method. However, only 30 of the patients met the required criteria to participate in all psychotherapy sessions. The participants were then randomly assigned to two intervention and control groups (15 participants per group). The intervention group received eight 120-min sessions of positive psychotherapy on a weekly basis while the control group was put on the waiting list. Data collection tools included the Oxford Happiness-Depression Questionnaire and Values in Action-Inventory of Strengths. The participants completed the questionnaires before and after the intervention. Data analysis was conducted by analysis of covariance. RESULTS: The results of covariance analysis suggested that the use of positive psychotherapy has been effective on increasing happiness (F = 0.129, P = 0.005), increasing character strength (F = 27.55, P = 0.001), increasing the semantic measures (F = 15.23, P = 0.002), increasing pleasure (F = 8.51, P = 0.005), and commitment (F = 64.19, P = 0.001) among patients with cancer. CONCLUSIONS: The results of the present study indicated that positive psychotherapy has been effective on happiness, strengths, pleasure, commitment, and semantic measures of the life among people with cancer. Positive psychotherapy can be used to minimize the effects of cancer on patients.