This article reports on the effect of textbook analysis as a tool of teacher professional development on nature of science (NOS) understanding of 10 science teachers in South Africa. The teacher professional development program (TPDP) was based on an explicit reflective methodology of textbook analysis and conducted online due to the Covid-induced lockdown. NOS understanding of the participant teachers was documented pre-training and post-training using a questionnaire designed by the researchers, termed the IFVNOS questionnaire. This tool was formulated based on the views of nature of science questionnaire version C (VNOSC) and the reconceptualised family resemblance approach (RFN) questionnaire. The same tool was used pre- and post-training. A comparison was made of the pre- and post-training results and it was found that there was a general individual increase in NOS understanding in 9 of the 10 teachers. The creative, scientific knowledge, science methods and ethical practices NOS aspects showed the greatest improvement in understanding by the teachers as a collective, whilst inferential NOS showed no overall change in understanding. This study showed that textbook analysis can be used as a professional development tool to improve NOS understanding of in-service science teachers.