Alexithymia is associated with increased depressive symptoms in both clinical and community samples. One way that alexithymia may lead to depression is through its impact on interpersonal relationships. Individuals with alexithymia report lower perceived social support, intimacy, and relationship satisfaction. Furthermore, poor relationship functioning is a clear risk factor for depressive symptoms. Given the established alexithymia–depression link and marital dysfunction–depression link, a logical next step is to examine whether relationship dysfunction (low social support, intimacy, negative relationship behaviours, and relationship dissatisfaction) mediates the association between alexithymia and depressive symptoms. The hypothesized mediation model was assessed in a sample of 104 community couples with two analytical approaches—first with cross‐sectional measures using path analysis and second with daily diary measures collected over a seven‐day period using a multilevel modelling approach. Poor relationship functioning mediated the association between alexithymia and depressed mood in the daily diary data and partially mediated that association with the cross‐sectional measures. These results identify alexithymia as an important variable in understanding the marital functioning–depression association, and this finding has implications for treatment. Copyright © 2012 John Wiley & Sons, Ltd.