Invasive micropapillary carcinoma of breast (IMpC) is a special type of breast cancer with frequent lymph node metastasis (LNM) and poor prognosis, while pure mucinous carcinoma of breast (PMC) is generally associated with infrequent LNM and better prognosis. A similar micropapillary epithelial growth pattern has been described in PMC that was named as invasive micropapillary mucinous carcinoma (IMpMC), but its prognostic significance is as yet not known. A retrospective review of 531 cases of PMC in 43,685 cases of breast cancer diagnosed over a 10-year period was conducted to assess the frequency of IMpMC and its prognostic implications. IMpMC was identified in 134 (25.2 %) of the 531 PMC cases. Compared to conventional PMC (cPMC), IMpMC was found more frequently in younger patients and in tumors with increased frequency of LNM and lymphovascular invasion, and higher HER2 expression. In stage-matched Kaplan-Meier analysis, patients with stage II-III IMpMC suffered a decreased overall survival and recurrence-free survival (RFS) than matched cPMC patients. Multivariate analysis confirmed the presence of IMpMC morphology was an independent unfavorable predictor for LNM and RFS of PMC. However, decreased LNM, lower nuclear grade, higher expression of ER and PR, less expression of HER2, and better prognosis were identified in IMpMC when compared with IMpC (n = 281). This is the first study to show the prognostic significance of IMpMC in a large cohort. IMpMC pursues a more aggressive clinical course than cPMC and should be managed differently; therefore, recognition of IMpMC and its accurate diagnosis are clinically important.