BackgroundBreast cancer is the second most common cancer in the world and also among Nepalese women. Breast self-examination is an important, cheap, and easy method for early diagnosis of breast cancer which can be cured in the majority of cases if diagnosed in the early stages. In developing countries like Nepal where the awareness regarding breast cancer and breast self-examination is poor, breast cancers are diagnosed at late stages resulting in a poor prognosis of the disease. The study assessed knowledge, practice, and factors associated with breast self-examination.MethodsA cross-sectional survey was carried out among 262 women in the Butwal sub-metropolitan adopting multi-stage sampling. A pre-tested structured interview schedule and an observation checklist were used to collect the data. Data was entered in EPI-data and necessary univariate, bivariate, and multivariate analyses were done in SPSS.ResultsThe study found that more than half of the participants (55.3%) had poor knowledge of BSE. Only one-fourth (27.1%) of them were practicing BSE and among them, most of them (93.0%) had poor practice. The factors such as ethnicity from Brahmin/Chhetri [AOR = 2.099, 95% CI (1.106–3.981)], use of contraceptive devices [AOR = 9.487, 95% CI (2.166–41.558)], personal history of breast lump [AOR = 12.502, 95% CI (1.639–95.387)], family history of breast cancer [AOR = 5.729, 95% CI (1.337–97.512)], and knowledge of BSE [AOR = 4.407, 95% CI = 2.160–34.650)] were significant determinants of BSE practice among 20–49 years women.ConclusionThe study concluded that most of the women had poor knowledge and practice of breast self-examination. The study also indicated the influence of ethnicity, contraceptives, personal and family history of cancer/early warning signs, and knowledge for practicing breast self-examination. There is an immediate need to increase the knowledge and practice of breast self-examination to prevent and detect breast cancer in its early stage.