Despite the evidence supporting reciprocal peer observation (RPO) for teacher professional development, extant literature has reported that teacher resistance is a barrier to its implementation. Few studies have analysed the variables that moderate resistance to RPO. Based on data from 394 in-service teachers who participated in an RPO intervention, three aims are addressed: (1) to examine pretest-posttest differences in resistance to RPO, (2) to analyse the effect of demographic variables and perception of school support in initial resistance to RPO, and (3) to analyse the effect of demographic variables and perception of school support and role performance in final resistance to RPO, after controlling for initial resistance. The Teachers' Resistance to Reciprocal Peer Observation Scale was validated and used as pretest-posttest. Student's paired sample t-test was carried out to compare the two scores, and linear regression was used for the second and third aim. Firstly, results show a significant decrease in resistance to RPO. Secondly, initial resistance to the observer and observee role is moderated by teaching experience and feeling properly trained, with additional variables moderating resistance towards being observed (i.e., gender, being part of the school management team, prior experience, school time arrangement). Thirdly, final resistance to both roles is moderated by initial resistance, teaching experience, and variables referring to judging oneself and the partner as capable of successfully participating in the intervention. Limitations and implications for research and practice are discussed. The changes in the quality of teaching that RPO may generate are contingent on reducing teacher resistance.