A father of a child with cancer experiences psychological stress related to his child’s cancer diagnosis and treatment, which may affect his relationship with his spouse. Nevertheless, little attention has been paid to how having a child with cancer affects the marital relationship from the perspective of the father. We examined the impact of the child’s cancer on the father’s relationship with his spouse in Korea. We conducted in-depth interviews with 20 fathers (mean age = 41.35 years; SD = 4.49) of children who were diagnosed with cancer before the age of 19 and were within 5 years of the diagnosis. The mean age of the child with cancer was 9.1 years (SD = 3.68), and the mean age at diagnosis was 6.4 years (SD = 4.08). The analysis of the interviews revealed the following four themes (and eight subthemes): conflicts between spouses (lack of father’s participation in caregiving, financial and work-related stress), mental suffering (heartbroken, torn between caregiving and work), change in communication (child-focused communication, avoiding communication), and change in the marital relationship (neglected relationship, new trust built in the relationship). A father’s increased conflict in the marital relationship after his child’s diagnosis of cancer was intensified by his limited involvement in child care and parenting responsibilities. An understanding of the change in the father’s relationship with his spouse can inform the development of a psychosocial intervention that may strengthen a father’s emotional intelligence and resilience, which could improve the marital relationship.