Introduction: Unplanned pregnancy is an important public health problem both in developing and developed world, because of its association with adverse social and health outcome for mothers, children and family as whole. The aim of this study was to assess the prevalence and associated factors of unplanned pregnancy among antenatal care attendants in Shashemane public hospitals, west Arsi zone in Oromia region. Methods: Institution-based cross sectional study design was employed on 380 pregnant women who came for antenatal care visit in Shashemane referral hospital and Melkaoda hospital from Mar 25- April 22/2019 G.C. The study participants were selected using systematic random sampling method and semi structured questionnaire was used to collect the data. The collected data were entered into Epi-data version 3.1 and exported to Statistical Package for Social Science version 22. Both bivariate and multivariable logistic regression analysis were performed to identify associated factors. P values <0.05 with 95% confidence level were used to declare statistical significance. Result: The prevalence of unplanned pregnancy was 31.1%. Most of the respondents (57%) were found to have poor knowledge towards family planning. Predictor like parity of three and above and having poor knowledge towards general use of family planning were found to be positively associated with unplanned pregnancy as evidenced by statistical result of [AOR=9.7 (95%CI):(2.0, 47.7)] and [AOR=2.85 (95%CI):(1.54, 5.3)] respectively. Being rural in residence [AOR =0.27 (95%CI) :( 0.15, 0.5)] and husbands educational level of primary and above [AOR=0.34 (95%CI): (0.16, 0.73)] were found to be protective factors for unplanned pregnancy. Conclusion: According to this study the prevalence of unplanned pregnancy was 31.1%. Maternal age group of 35- 45, rural residence, multiparty, and having poor knowledge towards general use FP are significantly associated with unplanned pregnancy.