Background: Breast cancer patients who undergo mastectomy encounter numerous problems, the most annoying of which is lymphedema followed by pain and decreased function in the affected limb. Objectives: This study examined the effect of self-care training on upper limb function and pain after breast surgery. Methods: This quasi-experimental study was performed on two groups of 60 patients with breast cancer in the Oncology Ward of Zahedan University of Medical Sciences in 2021. The patients were selected based on the inclusion criteria and through convenience sampling and were then randomly divided into intervention and control groups. The patients in the intervention group attended self-care training and exercise programs implemented in five sessions in addition to the routine care. One and three months after the intervention, upper limb function and pain were measured with DASH and McGill pain questionnaires. The repeated measures analysis of variance (ANOVA) and Bonferroni test were used to compare the pre-, and post-intervention mean scores and mean differences in the two groups. Results: The mean scores of upper limb function one and three months after the training program in the intervention group were lower than the mean scores of the control group. In other words, the quality of upper limb function was not significantly different despite the changes in the first month, but upper limb function significantly improved three months after the intervention (P < 0.001 vs. P = 0.06). The mean pain scores before, one month, and three months after the intervention in the intervention group were 10.4, 35.7, and 6.26, respectively, and the corresponding values in the control group were 10.8, 41.7, and 21.1, respectively, showing significant differences between the two groups, with the intervention group having lower pain scores than the control group (P = 0.001). Conclusions: Since lymphedema and its consequences, including decreased upper limb function and pain, are very serious issues, medical staff can give priority to this training program and implement it to prevent and control these complications.