Introduction: Human immunodeficiency virus, hepatitis B virus, and syphilis infections are arnwidespread problem among pregnant women around the world, with difficulties that are passed onrnto their newborn babies. In Ethiopia, STIs such as syphilis, HBV, and HIV are major public healthrnissues that result in a wide range of adverse pregnancy outcomes.rnObjective: The study aimed to determine the seroprevalence and associated factors of the humanrnimmunodeficiency virus, hepatitis B virus, and Syphilis infection and associated factors amongrnpregnant mothers attending antenatal clinics at governmental hospitals in Addis Ababa, Ethiopia,rn2021.rnMethod: An Institutional based cross-sectional study design was employed among 281 mothersrnfrom February to March 2021. Systematic random sampling technique was employed to select thernstudy participants and socio-demographic data, hospital-related factors, and risky socio-culturalrnand behavioral factors were collected by face-to-face interview using a structured questionnaire.rnSeroprevalence of HIV, HBV, and Syphilis was taken from the routinely prescribed laboratory testrnresults of the pregnant mothers. Data was entered into Epi-data 3.1, and SPSS version 25.0 wasrnused to modify, clean, and analyze it. Descriptive statistics, bivariate and multivariable logisticrnregression analysis were used. Variables with a P-value of < 0.05 were considered determinantrnfactors for HIV, hepatitis B virus, and syphilis infection.rnResult: Human immunodeficiency virus, hepatitis B virus, and syphilis seroprevalence were 5.3%,rn3.2 %, and 1.8% respectively. STIs (HIV, HBV, and Syphilis) were found in 26 (9.3%) of thernwomen, and 1.1% of the women had both HIV and syphilis co-infections. There were nornHIV/HBV, HBV/Syphilis and HIV/HBV/Syphilis co-infection. The history of multiple sexualrnpartners (AOR 3.42, 955 CI 1.0-11.63) and history of STIs (AOR 3.7; 955CI 1.07-13.45) werernsignificantly associated to HIV infection. Likewise, history of abortion (AOR 7.65, 95% CI 1.1749.74),rntattooingrn(AORrn9.72,rn95%rnCIrn1.41-66.73)rnandrninvoluntaryrnsexrn(AORrn9.72,rn95%rnCIrn1.4166.73)rnwerernsignificantlyrnassociaternwithrnhepatitisrnBrnvirusrninfection.rnHusbandrnhistoryrnofrnmultiplernrnsexualrnpartnerrn(AORrn20.9,rn95%rnCIrn1.8-241.8)rnwasrnsignificantlyrnassociaternwithrnsyphilisrninfection.rnrnConclusionrnandrnrecommendation:rnTherernarernstillrnarnhighrnprevalencernofrnhumanrnimmunodeficiencyrnrnvirus,rnhepatitisrnBrnvirus,rnandrnsyphilisrninrnourrncountryrnEthiopia.rnTherefore,rnitrnisrnimportantrntornscreenrnrntheserninfectionsrnamongrnallrnpregnantrnwomen atrneveryrnantenatalrncarernvisit.