Introduction: Sexually transmitted diseases are among the most common illnesses in thernworld and have far reaching health, social and economic consequences. University studentsrnare young adults and vulnerable to sexually transmitted diseases due to exposure riskyrnIXrnsexual behaviors such as having multiple sexual partners, inconsistent condom use, havingrnsex with commercial sexual partners and casual sexual partners, having sex after alcoholrnand having sex for gift or benefit. It is fundamental to understand the magnitude of riskyrnsexual behaviors and its effect on sexually transmitted diseases among students of AddisrnAbaba University in wider perspectives so that the results could be used to take appropriaternmeasures by different stakeholders involved in reproductive health programs.rnObjective: To assess the magnitude of risky sexual behaviors and its effect on sexuallyrntransmitted diseases among undergraduate students of Addis Ababa University in 2016rnMethodology: The study was conducted in all 10 colleges of Addis Ababa University onrnundergraduate students. A cross sectional study was used on 422 undergraduate studentsrnfrom March 2016 to April 2016.Multi- stage cluster sampling was used to select therncolleges. The sample size of the study subjects was calculated by proportion to populationrnsize from each college. Systematic random sampling technique was applied to select thernstudy participants. Data was entered and cleaned using EPI Info version 3.5.3 statisticalrnsoftware and exported to SPSS 21 for analysis. Odds ratio with 95% confidence intervalrnwas estimated to identify predictors of self-reported sexually transmitted diseases and riskyrnsexual behaviours.rnResult: A total of 422 undergraduate students were enrolled in the study with response raternof92% where 245 (63 %) were males. The median of age distribution was 21 years. Thernaccommodation of 251 (64.7%) students was in the universityâ€™s dormitory. One hundredrnfifty-eight students were sexually active in the past 12 months. Among the sexually activernstudents 92(58.2%) had more than one sexual partner in the past 12 months and forty-tworn(23.3%) never used condom in their lives. The students who reported experiencingrnsexually transmitted diseases were 18%(95%CI; 14.2, 22.2).The Multivariate analysisrnindicated that the students who had multiple sexual partners and inconsistent condom usersrnwere 2.69 and 2.38 times likely to develop sexually transmitted diseases respectively.rnConclusion: There is high burden of STDs among undergraduate students of AAU.rnMagnitude of STDs was affected by having multiple sexual partners and inconsistentrncondom use among undergraduate students of AAU. Students experienced risky sexualrnbehaviors such as having multiple sexual partners, inconstant condom use, having sex forrnbenefit/gift, having casual sexual partners and having sex with commercial sex workers.rnPredisposing factors such as peer pressure, watching pornography Medias, going tornnightclubs and having depression affected the magnitude of risky sexual behaviors amongrnthe study subjects.