Objective: The Serious Illness Care Program was developed to support goals and values discussions between seriously ill patients and their clinicians. The core competencies, that is, the essential clinical conversation skills that are described as requisite for effective serious illness conversations (SICs) in practice, have not yet been explicated. This integrative systematic review aimed to identify core competencies for SICs in the context of the Serious Illness Care Program. Methods: Articles published between January 2014 and March 2023 were identified in MEDLINE, PsycINFO, CINAHL, and PubMed databases. In total, 313 records underwent title and abstract screening, and 96 full-text articles were assessed for eligibility. The articles were critically appraised using the Joanna Briggs Institute Critical Appraisal Guidelines, and data were analyzed using thematic synthesis. Results: In total, 53 articles were included. Clinicians' core competencies for SICs were described in 3 themes: conversation resources, intrapersonal capabilities, and interpersonal capabilities. Conversation resources included using the conversation guide as a tool, together with applying appropriate communication skills to support better communication. Intrapersonal capabilities included calibrating one's own attitudes and mindset as well as confidence and self-assurance to engage in SICs. Interpersonal capabilities focused on the clinician's ability to interact with patients and family members to foster a mutually trusting relationship, including empathetic communication with attention and adherence to patient and family members views, goals, needs, and preferences. Conclusions: Clinicians need to efficiently combine conversation resources with intrapersonal and interpersonal skills to successfully conduct and interact in SICs.