IntroductionUnder‐five mortality reduction due to pneumonia is not Signiant, particularly in developing countries. Pneumonia contributed to 27.5% to 31.3% of health facility visits by children 2 to 59 months in Aleta Wondo Woreda. Previous studies have shown inconclusive evidence on determinants of pneumonia in children.MethodsAn institution‐based unmatched case–control study was conducted to assess determinants of pneumonia among under‐five children at Aleta Wondo Woreda, Sidama Region.ResultOne‐hundred forty‐five cases and 290 controls of children aged 2 to 59 months participated in the study. The mean ± (SD) age of the children was 18.81 months (2.1 ± 11.43) and 28.26 months (2.1 ± 16.007) for cases and controls, respectively. Only 56% (n = 145) of cases open house windows daily, whereas most 68.6% (n = 290) of controls house windows open daily. Ninety five (62.8%) of cases and 68.6% of controls were exclusively breastfed for 6 months. Household income ≥1500 Ethiopian birr (AOR = 0.45, 95% CI, 0.017–0.120, p < 0.000), child location outside of cooking house during cooking (AOR = 0.101, 95% CI, 0.43–0.238, p < 0.000), no formal education of the mother (AOR = 2.398, 95% CI, 1.082–5.316, p < 0.031), and presence of history of upper respiratory tract infections (URTIs) in last 2 weeks (AOR = 2.183, 95% CI, 1.684–5.273, P < 0.049) were determinants of pneumonia.ConclusionDeterminants of pneumonia in under‐five children were multifactorial (socioeconomic, nutritional, and environmental). Addressing these factors by involving all relevant stakeholders is important to reduce pneumonia‐related morbidity and mortality among under‐five children in the study area.