Background: To investigate the function of MAPT antisense RNA 1 (MAPT-AS1) in breast cancer (BC), evaluating its potential diagnostic value for BC patients. Methods: This study involved 498 patients with BC, 222 patients with benign breast diseases, including 74 cases each of of breast fibroadenoma, breast intraductal papilloma, and ductal hyperplasia and 429 healthy controls. We collected 20 cases of BC tissues and adjacent normal tissues and blood was taken from each participant. The expression levels of MAPT-AS1 were detected using reverse transcription polymerase chain reaction (RT-PCR). According to the median serum level of MAPT-AS1, patients were divided into a high expression group and a low expression group. We established MAPT-AS1 overexpression and knockdown in human BC cell line ZR-75-1 and MDA-MB-231 to study the function on cell proliferation [using cell counting kit-8 (CCK-8) assay], migration, and invasion (using Transwell assay). Results: The expression of MAPT-AS1 was significantly up-regulated in tissues and serum of BC patients compared with controls (P<0.0001 for both). Receiver operating characteristic (ROC) curve analysis showed that MAPT-AS1 had a large area under the curve (AUC) of 0.893. The expression of MAPT-AS1 in serum was closely related to the large tumor size, grade, tumor-node-metastasis (TNM) stages, and human epidermal growth factor receptor 2 (HER-2) expression status of BC patients. Overexpression of MAPT-AS1 activated the Wnt/beta-catenin signaling pathway, promoting proliferation, migration, and invasion. Conclusions: Overexpression of MAPT-AS1 in tissues and serum is a reliable diagnostic marker for BC, and MAPT-AS1 regulates the proliferation and metastasis of BC cells by activating the Wnt/beta-catenin signaling pathway.