Objective: Lack of assertive behaviors has been considered a possible vulnerability to depression; the cross-sectional design was utilized in most past studies to investigate the relationship between assertive behaviors and depressive symptoms, which can not clarify whether non-assertiveness is a vulnerability to depression, or just a consequence of depression. Moreover, assertiveness is a concept from Western culture, and the relationship between non-assertiveness and depression for people who live in Chinese culture remains uncertain. This study used the remission and longitudinal designs to explore the relationship between assertive behaviors and depressive symptoms in patients who had been diagnosed with major depressive disorder in Taiwan. Method: (1) In the remission design, the participants included 30 patients who were under a major depressive episode currently, 21 patients with remitted major depressive disorder, and 30 normal controls. Assertive behaviors were measured by the Interpersonal Behavior Survey (IBS), including four assertive behavior subscales (self-confidence, initiating assertiveness, defending assertiveness, and frankness). Depressive symptoms were assessed by the Beck Depression Inventory-II (BDI-II). (2) In the longitudinal design, the depressive symptoms of both depressed and remission patients were reassessed after two months from the first assessment. Thirty participants completed the BDI at the reassessment. Results: (1) The depressed group demonstrated lower scores than the controls did on four assertive behavior subscales, while the remission and control groups did not show significant difference in the four subscales. (2) After controlling the first BDI assessment scores, self-confidence subscale still significantly predicted the second BDI assessment scores. Conclusion: The results revealed that lack of interpersonal self-confidence predicted future depressive symptoms, which supported that lack of self-confidence could be a vulnerability to depression. For implication in clinical practice, therapists can help clients by focusing on their assertive behaviors and enhancing clients’ self-confidence and motivation for interpersonal interaction to relieve depressive symptoms.