Background Restrictive abortion laws are the single most important determinant of unsafe abortion, a major, yet preventable, global health issue. While reviews have been conducted on the extent of the phenomenon, no study has so far analysed the evidence of why women turn to informal sector providers when legal alternatives are available. This work provides a systematic review of the qualitative literature on informal sector abortion in setting where abortion is legal. Methods We used the PRISMA guidelines to search Pubmed, Web of Science, Sciencedirect and Google Scholar databases between January and February 2018. 2794 documents in English and French were screened for eligibility against pre-determined inclusion and exclusion criteria. Articles investigating women’s reasons for aborting in the informal sector in settings where abortion is legal were included. In total, sixteen articles were identified as eligible for this review. Findings were reported following the PRISMA guidelines. Results The review highlights the diverse reasons women turn to the informal sector, as abortions outside of legal health facilities were reported to be a widespread and normalised practice in countries where legal abortion is provided. Women cited a range of reasons for aborting in the informal sector; these included fear of mistreatment by staff, long waiting lists, high costs, inability to fulfil regulations, privacy concerns and lack of awareness about the legality of abortion or where to procure a safe and legal abortion. Not only was unsafe abortion spoken of in terms of medical and physical safety, but also in terms of social and economic security. Conclusion The use of informal sector abortions (ISAs) is a widespread and normalised practice in many countries despite the liberalisation of abortion laws. Although ISAs are not inherently unsafe, the practice in a setting where it is illegal will increase the likelihood that women will not be given the necessary information, or that they will be punished. This study brings to the fore the diverse reasons why women opt to abort outside formal healthcare settings and their issues with provision of abortion services in legal contexts, providing an evidence base for future research and policies. Electronic supplementary material The online version of this article (10.1186/s12905-019-0751-0) contains supplementary material, which is available to authorized users.