The purpose of this study is to examine the expression of Yes-associated protein (YAP) in metaplastic carcinoma and compare to those of triple-negative breast carcinoma (TNBC) for investigation of its implication. Tissue microarrays containing 34 cases of metaplastic carcinoma and 175 cases of TNBC were constructed and immunohistochemical staining was used to evaluate expression of the following proteins: YAP and phosphorylated YAP (pYAP). According to immunohistochemical staining results of cytokeratin 5/6, EGFR, claudin 3, claudin 4, claudin 7, E-cadherin, STAT-1, androgen receptor, and GGT-1, metaplastic carcinoma and TNBC were sub-classified into six subtypes: basal-like type, molecular apocrine type, claudin-low type, immune-related type, mixed type, and null type. Comparing the expression of YAP and pYAP in metaplastic carcinoma and TNBC, the expression of nuclear YAP (p = 0.025), cytoplasmic pYAP (p = 0.010), and nuclear pYAP (p = 0.014) in tumor cell was higher in metaplastic carcinoma than TNBC. In metaplastic carcinoma, the nuclear YAP expression in tumor cell was associated with loss of E-cadherin (p = 0.020) and claudin type (p = 0.020), and the stromal YAP expression was associated with claudin 7 positivity (p = 0.003). In conclusion, the YAP expression in metaplastic carcinoma is higher than that in TNBC, representing the association of stemness and epithelial-mesenchymal transition features in metaplastic carcinoma.