Background: In Ethiopia, there are different pocket studies that assessed adolescent undernutrition which came up with inconsistent and inconclusive findings. Therefore, estimating the pooled prevalence and associated factors of the adolescent undernutrition using meta-analysis is crucial in Ethiopia. Methods: A systematic review of eligible articles was conducted using preferred reporting items for systematic reviews and meta-analysis (PRISMA) guidelines. A comprehensive searching of the literature was made in Pub Med, Scopus, Google, Google Scholar, Cochrane Library and CINAHL. The quality of the articles included in the review was assessed using the Newcastle-Ottawa Scale (NOS) for non-randomized studies in meta-analyses. The pooled prevalence and odds ratio of the associated factors with their 95% confidence interval was computed using STATA version 14 software. Results: Twenty-two studies were included in the meta-analysis with a total of 17,854 adolescents. Using the random-effects model analysis, the pooled prevalence of stunting and underweight was 20.7% (95% CI: 16.08, 25.33) and 27.5% (95% CI: 17.9, 57.14), respectively. Rural residence, having family size≥5, households with an unprotected water source for drinking and food-insecure household were significant associated factors for adolescent stunting. Early adolescent age (10-14 years), family size≥5, food-insecure household, lack of latrine, WHO diet diversity score < 4, mother educational status (with no formal education) were significant associated factors for adolescent underweight. Conclusion: Adolescent undernutrition remains one of the most important public health problems in Ethiopia. Almost a quarter of Ethiopian adolescents were affected by stunting and underweight. Large family size, rural residence and unprotected source of drinking water were the associated factors for adolescent stunting. Similarly, large family size, early age of adolescent, lack of latrine, low dietary diversity score, mother illiteracy, and food insecure household were the associated factors for adolescent underweight. It would be good to give high emphasis on adolescent undernutrition and it is important to address the above mentioned associated factors during adolescent nutritional interventions in Ethiopia.