Since the 1990s, the rise of some online communities as well as the decline of others has caught the attention of academia as well as of practice. One assumption for the decline of some online communities is the lack of the rich knowledge content that is believed to be the source of competitiveness and sustainability of any online community. Online communities are increasingly acknowledging the value of knowledge and the knowledge sharing processes required for online communities to build and sustain their identity in this competitive and constantly changing online environment. This research aims to provide an understanding of knowledge sharing behavior through the adaptation of two major theories imported from Social Psychology: the Theory of Reasoned Action (TRA) and the Theory of Planned Behavior (TpB). Exploring knowledge sharing from a social psychological perspective provides an understanding of the process an individual goes through to make the decision to share his/her knowledge with others as well as the different psychological factors facilitating or hindering knowledge sharing behavior. Research findings are based on a web-survey of 158 group members, and an observation of interactions of eight active groups in one online community of professional educators. Proposing an extended theoretical model of knowledge sharing behavior in an online community, this research found that normative pressures, including subjective norms and descriptive norms, had a strong influence on the formation of the individual’s intention to share in the online community. Knowledge sharing self-efficacy also was found to significantly account for explaining the individual’s motivation to share his/her knowledge with other members. Attitude and controllability were not found to have significant impacts on the formation of intention. The qualitative analysis of the interactions of members of eight active groups revealed that there were other implicit factors that motivated individuals to engage in online activities. The observation of 24 online sessions had generated beliefs related to knowledge sharing behavior. Those beliefs were related to normative beliefs and the pressure created by the expectations of others, control beliefs and the confidence of the individual’s ability to behave, and finally, behavioral beliefs and individual outcome expectations.