Purpose: Knowledge-sharing is a valuable learning activity among teachers that leads to individual and collective professional development and contributes to students' learning outcomes and school effectiveness. However, teachers are generally weak at knowledge-sharing and are often professionally isolated from colleagues. Regardless of the general and context-specific reasons for this weakness, researchers believe that principals' knowledge-sharing leadership (KSL) can influence teachers' knowledge-sharing behaviours (KSB). Nevertheless, little is known about how it can exert its impact. In addition, given the precedence of teachers' beliefs and intentions over their behaviours, the mechanism of this impact has not yet been investigated based on this sequence. This study seeks to investigate and compare this impact in the form of two competing models, including the theory of reasoned action and planned behaviour. Both models consider KSB due to teachers' beliefs and intentions. Design/methodology/approach: In total, 445 teachers completed an online form comprising two scales, including KSL and the knowledge-sharing belief-intention-behaviour. Data were analysed using structural equation modelling (SEM). Findings: The model hypothesising the impact of KSL on constructs of the theory of reasoned action had a better fit with the data. The direct and indirect relationships analysis showed that KSL directly affects two fundamental beliefs of teachers, including attitudes toward knowledge-sharing (ATKS) and subjective norms (SN). Also, these beliefs directly affected teachers' intentions and, ultimately, their KSB indirectly. Originality/value: This study contributes to the school leadership literature. Further, it has supported the theory of reasoned action when the teacher's KSB is targeted. Although researchers previously have adopted this theory, they have focused more on containing constructs while neglecting principal-related external variables. Finally, this study provides insights into principals' training programs. While principals may try to be knowledge-sharing leaders through self-practising competencies, they should be purposefully trained.