Background Stromal reaction is an important prognostic factor in several cancers, and the presence of myxoid change was assessed as a poor prognostic factor in colorectal cancer. However, the prognostic significance of myxoid change in triple-negative breast cancer (TNBC) remains unknown. This study aimed to determine the prognostic significance of myxoid change and fibrotic focus (FF), which is a fibrotic area within the tumor and considered a poor prognostic indicator in patients with TNBC. Methods We enrolled 62 patients with TNBC and reviewed the surgically resected specimens to evaluate myxoid change and FF in the tumor using previously outlined criteria. We evaluated tumor-infiltrating lymphocytes (TILs) using hematoxylin and eosin slides. Overall survival (OS) and relapse-free survival (RFS) were compared based on the presence of myxoid change and/or FF, and the risk factors for RFS were analyzed. Results Myxoid change and FF were observed in 25.8% and 33.9% of specimens, respectively. Based on stromal lymphocyte infiltration, 19 patients (30.6%) had high TILs, while the remaining 43 patients (69.4%) had low/intermediate TILs. Presence of myxoid change was significantly correlated with poor OS and RFS (p = 0.040 and 0.031, respectively). FF was also significantly correlated with poor OS and RFS (p = 0.012 and 0.028, respectively). The combination of myxoid change and FF was an independent and poor prognostic factor according to the multivariate analysis (HR 11.61; 95% CI 1.027-131.2; p = 0.048). Presence of myxoid change and FF were significantly associated with low/intermediate TILs in the stroma (p = 0.013). Conclusions Histopathological assessment of myxoid change and FF in TNBC may be a useful, practical, and easily assessable method for predicting prognosis in patients with TNBC, which should be confirmed in larger prospective studies. Diagnostic criteria for the establishment of myxoid change and FF in TNBC must be established, and their underlying molecular events must be clarified.