Background Postnatal care services are a constellation of preventive care, practices, and assessments designed to identify and manage maternal and newborn complications during the first six weeks after birth. Recognizing the role of the appropriate PNC at this critical time, the World Health Organization recommended four visits as a complete PNC for all post-partum mothers and newborns to ensure their survival. Although there have been numerous studies on the factors affecting the general PNC service in Ethiopia, there is a shortage of evidence on the use of complete postnatal care services and its determinants. Therefore, the objective of this study was to assess the uptake of full postnatal care service and its determinants among women who recently gave birth in the Ezha district of southern Ethiopia. Methods A community-based cross-sectional study was conducted in Ezha district. A two-stage sampling technique was applied. A total of 568 mothers who stayed for at least 6 weeks after childbirth from selected Kebeles were included in the study using computer-generated random numbers. Data collected through pre-established, structured, and interviewer-administered questionnaires were entered into EpiData3.1 and exported to SPPS version 23 for analysis. The Principal Components Analysis (PCA) was carried out to assess the wealth status of participants. The Multivariable logistic regression model has been fitted to identify the determinants of complete post-natal care service uptake. Results The overall uptake of complete postnatal care services in the study area was 23.9% [95% CI: (19.9, 27.5)]. The factors namely; maternal education of secondary and above [AOR: 4.31, 95%CI: 2.15, 8.05], having four and more antenatal visits [AOR: 4.03, 95%CI: 1.83, 8.85], Caesarean delivery [AOR: 3.75, 95%CI: 1.78, 7.92], having good knowledge on PNC [AOR: 4.31, 95%CI: 2.34, 9.04], and being a model household [AOR: 3.61, 95%CI: 1.97, 6.64] were recognized as determinants complete postnatal care uptake. Conclusion and recommendation Complete post-natal care services uptake in the study area was low. Thus, a due emphasis should be given to behavioral change communication activities to improve maternal knowledge on PNC and enhancing adequate ANC uptake by health workers in the district. Besides, health extension workers in the district need to work on creating a model household through continuing education, support, and follow-up.