In knowledge management (KM)-related research, effective knowledge sharing is considered to be one of the most critical components of KM success. For the present research, the authors conducted a longitudinal, two-phased study to evaluate if the Theory of Reasoned Action (TRA) and three variations of the Theory of Planned Behavior—namely, TPB, decomposed TPB (DTPB), and revised TPB (RTPB)—can adequately predict knowledge sharing behaviors. The first TRA-based study shows a severe limitation in the ability of the intention to predict actual knowledge sharing behaviors collected from a knowledge management platform. In a subsequent study, three variations of TPB-based models were employed to show that, although the independent variables (i.e., attitude, subjective norm, and perceived behavior control that is decomposed into controllability and self-efficacy) give satisfactory explanations of variance in intention (R² > 42%), the intention–behavior gap still exists in each of the three models. Only the perceived self-efficacy in the revised TPB can directly predict knowledge sharing behaviors. This gap highlights the importance of knowledge sharing as a fundamentally social activity for which the actualization of intention into actions may be interrupted due to barriers such as a mistake-free culture or others’ deliberate misinterpretations that may in turn cause unanticipated negative consequences to the person. The theoretical implication of this study is that in applying TPB to study knowledge sharing practices, researchers must focus on control beliefs that reflect people’s capacity to overcome possible environmental challenges encountered in carrying out their knowledge sharing intentions.