Background: There is growing evidence that conversations between healthcare professionals and patients with serious illness can improve the quality of end-of-life cancer care. Yet, there is lack of insight into how different healthcare professions collaborate to deliver serious illness communication, as well as patients’ and caregivers’ perceptions of this collaboration between the nurse and physician. This study explores the interdisciplinary collaboration between nurses and physicians in serious illness conversations with patients diagnosed with multiple myeloma and their caregivers. Methods: Eleven dyadic interviews were conducted with 22 patients and caregivers, and two focus group interviews involving four nurses and the other with four physicians. Data analysis and reporting were conducted using reflexive thematic analysis within phenomenological epistemology. Results: The interdisciplinary collaboration was characterized by three main themes: (1) Importance of relationships, (2) Complementary perspectives, and (3) The common goal. Conclusion: This study highlights the importance of interdisciplinarity in serious illness conversations as it enhances the use of existential and descriptive language when addressing medical, holistic, and existential issues. The use of broader language also reflects that interdisciplinary interaction strengthens the expertise of each professional involved in patient care. Through interdisciplinary collaboration, the preferences, hopes, and values of the patient and caregiver can be integrated into the treatment plan, which is key in providing the delivery of optimal care. To promote cohesive and coordinated collaboration, organizational changes are recommended such as supporting continuity in patient–healthcare professional relationships, providing interdisciplinary training, and allocating time for pre-conversation preparation and post-conversation debriefing.