Objective To explore the subjective experiences and underlying mechanisms of delays in seeking medical care among adolescents with depression, and to provide evidence for the early identification of delayed healthcare seeking and the development of targeted nursing interventions. Methods A purposive sampling strategy was employed to recruit 13 adolescents with depression who had experienced delayed healthcare seeking. Semi-structured interviews were conducted, and the data were analyzed using content analysis. The findings were subsequently mapped to the Health Belief Model for theoretical interpretation. Results Four themes and 11 subthemes were identified. The first theme, vague illness perception and cognitive bias, comprised somatic expression and normalization attribution of symptoms, defensive and multifaceted symptom attribution, and catastrophic perceptions of disease severity. The second theme, perceived benefits and perceived barriers in healthcare-seeking decision-making, included idealized expectations of treatment benefits, multidimensional perceptions of barriers to healthcare seeking, and practical obstacles encountered along the healthcare-seeking pathway. The third theme, the dual influence of the social support system on healthcare-seeking behavior, encompassed the facilitating and hindering roles of social support, accessibility of professional resources and biases in information acquisition, and the triggering effect of cues to action on healthcare-seeking behavior. The fourth theme, dynamic changes in self-efficacy during healthcare seeking, included variations in the initiative of help-seeking behavior and the dynamic adjustment of health beliefs. Conclusion Delays in seeking medical care among adolescents with depression result from the interaction between multidimensional imbalances in Health Belief Model constructs and social-ecological factors. Nursing professionals can facilitate timely healthcare seeking by strengthening mental health education, enhancing family support interventions, and implementing continuous nursing management to improve adolescents′ awareness of disease-related risks and reduce barriers to healthcare seeking.