Trastuzumab is a standard treatment for HER2-positive (HER2+) breast cancer, but some patients are refractory to the therapy. MicroRNAs (miRNAs) have been used to predict therapeutic effects for various cancers, but whether miRNAs can serve as biomarkers for HER2+ metastatic breast cancer (MBC) patients remains unclear. Using miRNA microarray, we identify 13 differentially expressed miRNAs in the serum of HER2+ MBC patients with distinct response to trastuzumab, and four miRNAs are selected to construct a signature to predict survival using LASSO model. Further, our data show that miR-940 is mainly released from the tumor cells and miR-451a, miR-16-5p and miR-17-3p are mainly from the immune cells. All these four miRNAs directly target signaling molecules that play crucial roles in regulating trastuzumab resistance. In summary, we develop a serum-based miRNA signature that potentially predicts the therapeutic benefit of trastuzumab for HER2+ MBC patients and warrants future validation in prospective clinical trials.