BACKGROUND: Undernutrition denotes inadequate intake of energy and nutrients to meet one's optimal needs. Pregnant women are at an increased risk of undernutrition due to the added nutritional demand for fetal growth and development. Updated evidence is important for monitoring progress and informing healthcare decisions. However, there is a paucity of evidence about the nutritional status of pregnant women in the rural communities of eastern Ethiopia, particularly in Chiro district. Therefore, this study aimed to assess the prevalence of undernutrition and its associated factors among pregnant women in Chiro district, eastern Ethiopia. METHODS: A community-based cross-sectional study was employed in Chiro district, from November 1 to 30, 2022, among 423 randomly selected pregnant women. The data were collected using face-to-face interviews and anthropometric measurements by trained research assistants. Bi-variable and multivariable logistic regressions were fitted to identify predictors of undernutrition by using STATA-14 statistical software. RESULTS: The overall prevalence of undernutrition was 40.9% (95% confidence interval (CI): 36.2%, 45.8%). Antenatal care (ANC) follow-up (adjusted odds ratio (AOR): 5.61, 95% CI: 3.01, 10.45), history of illness in the past 2 weeks (AOR: 4.25, 95% CI: 2.23, 8.12), khat chewing (AOR: 4.01, 95% CI: 2.12, 7.56), household food security (AOR: 5.84, 95% CI: 3.36, 10.14), and dietary diversity practice (AOR: 2.74, 95% CI: 1.47, 5.09) were factors associated with undernutrition. CONCLUSION: Four out of every 10 pregnant women in the study area were undernourished. ANC follow-up, illness in the past 2 weeks, khat chewing, household food security, and dietary diversity practice were significantly associated with undernutrition. Therefore, improving access to ANC services, strengthening nutritional counseling, and addressing food insecurity might reduce undernutrition among pregnant women.