PurposeTo examine the effect of diaphragmatic breathing (DB) on nausea, vomiting, and functional status among breast cancer patients undergoing chemotherapy in Indonesia. MethodsA quasi experimental study with non-equivalent pretest and posttest control group was conducted. A total of forty-eight breast cancer patients (24 DB and 24 control participants) undergoing chemotherapy participated in this study selected conveniently. DB intervention was performed to the intervention group after chemotherapy cycle twice a day for six days, meanwhile the control group received usual care. A set of questionnaires was used to collect data consisting of Patient Information Form, Rhodes Index Nausea, Vomiting and Retching (RINVR) and The Functional Living Index-Cancer (FLI-C). Data were analyzed and interpreted using Generalized Linear Model, Wilcoxon Test, Paired T Test and Mann-Whitney U test. ResultsThere were significant changes in RINVR mean scores for the intervention group that started on the third day after chemotherapy (p = 0.000); meanwhile, the significant changes in RINVR mean scores for the control group began on the fifth day (p = 0.000). The total score of FLI-C was significantly different between the intervention and control groups (p = 0.000). ConclusionDB could decrease nausea and vomiting, and increase functional status of breast cancer patients undergoing chemotherapy. It can be promoted as a useful low-cost self-management approach and an additional and complementary therapy to manage chemotherapy-related nausea and vomiting.