Depression and anxiety are the most common mental illnesses in the U.S., affecting millions annually. Attribution theory posits that illnesses seen as controllable (e.g., psychological) tend to be harshly stigmatized. Sufferers of depression and anxiety often experience negative social consequences such that they seek support from, irritate, and become rejected by others cyclically. Symptoms affect dating/marital relationships through conflict, intimacy, and satisfaction. Fifty-nine undergraduates were recruited in the present study from psychology and communications courses (SONA) after reporting at least mild depressive (n = 11), anxious (n = 30), or comorbid (n = 18) symptoms and endorsing being in a romantic relationship. Researchers collected corresponding data from romantic partners (RPs). Overall, endorsement of biological/uncontrollable or psychological/controllable sources of symptoms was related to lower support. Partner endorsement of psychological/uncontrollable causes was related to lower depth. For depressive symptoms, self- and partner-endorsement of biological/uncontrollable sources was related to lower conflict ratings. Greater endorsement of psychological/controllable sources was related to higher conflict. For anxious symptoms, greater endorsement of controllable sources was related to higher conflict. Partner agreement was related to higher depth and support, however, higher disagreement was only related to higher depth. SONA participants assumed similarity (bias) between their RPs' beliefs about all uncontrollable causes and their own but were only accurate in assuming similarity for most controllable causes. These findings suggest that attributions about symptoms operate in a complex way within couples, both deleteriously by predicting higher conflict and lower support while also relating to greater depth. Prior research has shown socially-hindering perceptual biases in anxious participants, but in the present study, participants with anxious symptoms were slightly more accurate than those with depressive symptoms in judging their RPs' beliefs about illness. Relatedly, the mixed findings for depressive symptoms suggest that attributions about illness controllability play an important role in how symptoms are perceived by both sufferers and their romantic partners. These results have implications for research on stigma of mental illness as well as within social relationships and contribute to a multifaceted understanding of how symptoms of the two most prevalent psychological illnesses may influence an important close relationship.