Background:The World Health Organization, the United States Society for Maternal-Fetal Medicine, the American College of Obstetricians and Gynaecologists, the Academy of Breastfeeding Medicine, the Food and Drugs Administration, and the Centers for Disease Control recommend COVID-19 vaccination to lactating mothers without discontinuity of breastfeeding. Despite this recommendation, willingness to receive COVID-19 vaccine and its determinant factors among lactating mothers were not studied in Ethiopia. Hence, this study aimed to assess willingness to receive COVID-19 vaccine and its determinant factors among lactating mothers in Ethiopia. Methods: An institutional-based cross-sectional study design was employed in southern Ethiopia from February 1 up to March 15, 2021. Multistage sampling technique was used to select study participants. Structured and face-to-face interviewer-administered questionnaire was used to collect data. Data were clean, coded, and entered into Epi-Data version 4.2.0 and exported to SPSS version 23 software package for analysis. Bivariable and multivariable analysis were used to identify associated factors. Adjusted odds ratio along with 95% CI was used to measure the strength of association. Results: The prevalence of willingness to receive COVID-19 vaccination was found to be 61% (95% CI; 56.9-65.1%). Urban residence [AOR=2.5, (95% CI; 1.62-3.91)], having secondary and above maternal educational status [AOR=2.8, (95% CI; 1.51-4.21)] mothers who had got immunization counselling [AOR=3.4, (95% CI; 1.95-5.91)], good knowledge about vaccine [AOR=2.6, (95% CI; 1.84-3.47)], and good adherence to COVID-19 mitigation measures [AOR=3.2, (95% CI; 1.91-5.63)] were determinant factors of willingness to receive COVID-19 vaccine. Conclusion: Urban residence, secondary and above maternal education status, immunization counselling, good knowledge about the vaccine, and good adherence to COVID-19 mitigation measures were determinant factors of willingness to receive COVID-19 vaccine. Therefore, health professionals should counsel about the merits of COVID-19 vaccination, and enhance maternal awareness about the vaccine is recommended.