Background: Toxoplasmosis is serious in the case of immune suppression and prenatal transmission. In immunocompromised hosts, it is manifested primarily as a life-threatening condition, toxoplasmic encephalitis. Congenital toxoplasmosis results in abortion or congenitally acquired disorders which primarily affect the central nervous system. This study assessed seroprevalence of Toxoplasma gondii (T. gondii) infection and associated factors among HIV-infected women within the reproductive age group (18-49 years) at Mizan Aman General Hospital, Southwest Ethiopia. An institution based cross-sectional study was conducted from February 01 to May 30, 2015. Systematic random sampling technique was employed for participant selection. Enzyme linked immuno sorbent assay was used to test for T. gondii from venous blood specimens. Participants were interviewed using structured questionnaire for different variables. Descriptive statistics, binary and multivariable logistic regression analyses were performed during data analysis. P value of less than 0.05 was considered statistically significant., Results: A total of 270 HIV-infected women within the reproductive age group were included in the study. Mean age of the respondents was 31 years (SD = ±6.5). Of the total study participants, 255 (94.4%), 95% CI (91.6, 97.2%) were found to be seropositive for T. gondii anti-immunoglobulin G (IgG) antibody, and 6 (2.2%), 95% CI (1.3, 3.1%) for anti-immunoglobulin M (IgM). All the anti-IgM positive samples were also positive for IgG. Multivariate analysis showed that; age within 28-37 years (Adjusted Odds Ratio [AOR] 2.58, 95% CI 1.01, 6.60), level of education with unable or only able to read and write (AOR = 4.46, 95% CI 1.20, 16.60), and substance abuse (AOR = 4.49, 95 CI 1.60, 12.55) were significantly associated with seropositivity of T. gondii infection., Conclusions: Seroprevalence of toxoplasmosis among the HIV-infected women in the childbearing age group in Mizan Aman was high. Age, educational status and drug addiction were identified as factors associated with T. gondii infection. Education of HIV-infected women about the transmission and prevention methods of T. gondii infection is important. Besides, studies on incidence of toxoplasmosis in newborns and infants are recommended.