Triple-negative breast cancer (TNBC) is a common subtype of breast cancer among women with dense breasts and is associated with a poor prognosis. Diet is a modifiable risk factor for breast cancer, but the association between the risk of TNBC and diet has not been studied. The present study investigated the hypothesis that a healthy dietary pattern was negatively associated with risk of TNBC in women with dense breasts (density types 3 and 4). After total mastectomy or breast conserving surgery, patients with TNBC (n = 38) and non-TNBC (n = 178) were recruited. The risk of TNBC was negatively associated with a healthy dietary pattern involving consumption of seafood and vegetables, and positively associated with a dietary pattern of consuming grains, meats, and eggs. Additionally, intake of animal fat, eggs, and meat was positively associated with the risk of TNBC, but intake of vegetable fats, nuts, and vegetables showed a negative association. Patients with TNBC had higher proportion of obesity or abdominal obesity than those with non-TNBC. The present study is the first to show a negative association between risk of TNBC and a healthy dietary pattern in women with dense breasts and suggests the importance of diet in the prevention of TNBC.