BACKGROUND: Unsafe abortion is now a global agenda because 45% of all global abortions are unsafe, and 97% are occurring in developing countries. In Tanzania, one million reproductive-aged women face unplanned pregnancies per year, and 39% end up with abortion. About 16% of maternal deaths are reported per year in Tanzania, and unsafe abortion takes the second position. There are several efforts to prevent and intervene unsafe abortions, such as equipping healthcare facilities across all levels of healthcare, approval of Misoprostol use, establishment of comprehensive post-abortion care (PAC), revising policy guidelines and standards, provision of emergency contraceptives, and capacity building of healthcare providers. There is little documentation about how the constructs of the theory of planned behaviour, knowledge, and sociodemographics influence the practice of abortion. OBJECTIVES: To assess the association of knowledge level, sociodemographic characteristics, and constructs of the theory of planned behaviour (TPB) to the practice of unsafe abortion among postnatal mothers at Mkonze Health Center in the Dodoma region. METHODOLOGY: It is an analytical cross-sectional study design conducted in Dodoma-Tanzania and involved 206 postnatal women. A validated questionnaire was used and analysis was performed in the Statistical Package for the Social Sciences (SPSS), through descriptive and inferential statistics. RESULTS: The practice of unsafe abortion in the current study is 28/206 (13.6%), influenced bytheir lower educational level and being single women. It was found that the majority had adequate knowledge of unsafe abortion 129 (62.6%), positive attitude 130 (63.1%), good subjective norms 113 (54.9%), and positive perceived behavioral control111 (53.9%). Knowledge, attitude, subjective norms, and perceived behavioral control were not significantly associated with the practice of unsafe abortion. CONCLUSION: The majority of the respondents had high knowledge, attitudes, subjective norms, and perceived behavior control on unsafe abortion. This is an indicator that the implemented initiatives are effective. Maintaining the ongoing effort and improving strategies are promising to mitigate the burden of unsafe abortion. Future research needs to find out hidden factors associated with attitude and how health beliefs might influence someone's attitude towards unsafe abortion.