This study examines the informal learning undertaken by secondary school science teachers, in order to enhance their professionality. Situated within the field of science education, this research focuses on the role of informal learning in science teachers’ professional development (PD). While there is a substantial body of research examining teachers’ formal PD, there has been limited research on science teachers’ informal learning as a form of PD. This study aims to broaden our understanding regarding the ways by which teachers develop their professionality across their career. Specifically, the study posed the following questions: How frequently are secondary school science teachers presently accessing informal learning resources? What are the teachers’ purposes in accessing informal learning resources? And how do the number of years teaching experience relate to teachers’ usages of the various informal learning resources? A mixed methods approach was applied to the collection and analysis of data from two sources: semi-structured questionnaires with secondary school science teachers (n = 91); and two focus group sessions – one with biology teachers (n = 11), and one with chemistry teachers (n = 6). Teachers identified the frequency and purposes of accessing 32 resources. The various resources were grouped into the following four categories: interactive media resources, non-interactive media resources, interpersonal communications, and exhibitory and experiential learning resources. The findings reveal specific preferences for types of resources, and clear purposes for accessing informal learning. In regard to the preferred resources, teachers prioritise the use of non-interactive media resources, such as Google and YouTube, over interactive media resources such as Facebook and Twitter; and, resources which provide ‘tailored’ information, such as Google over resources which provide general, non-specific, information such as newsletters and online courses. The most highly prioritised source of informal learning is conversations with colleagues. Online interactive communications do not appear to be a substitute for face-to-face contact. No significant differences were found in the frequency with which teachers with a different number of years of science teaching experience, access the various informal learning resources. In regard to the purposes of accessing informal learning, the study reveals two distinguishable needs: the first is as a means for developing professional identity, and the second purpose is to aid teaching. The discussion of the findings highlights the need for providing and promoting opportunities for face-to-face, peer conversations among teachers, and that these face-to-face conversations cannot be replaced by online alternatives. It further suggests that informal learning contributes not only to enhancing the teachers’ science teaching capabilities but also to the lifelong development of their professional identity. This study provides practical implications suggesting that accreditation bodies need to find more accurate indices for capturing the ways in which teachers enhance their professionality. Similarly, principals, and organisers of PD need to develop more awareness regarding the informal PD needs of science teachers and to facilitate these accordingly.