Objective Prostate cancer and its treatments, particularly androgen deprivation therapy (ADT), affect both patients and partners. This study assessed how prostate cancer treatment type, patient mood, and sexual function related to dyadic adjustment from patient and partner perspectives. Methods Men with prostate cancer (n = 206) and partners of men with prostate cancer (n = 66) completed an online survey assessing the patients' mood (profile of mood states short form), their dyadic adjustment (dyadic adjustment scale), and sexual function (expanded prostate cancer index composite). Results Analyses of covariance found that men on ADT reported better dyadic adjustment compared with men not on ADT. Erectile dysfunction was high for all patients, but a multivariate analysis of variance found that those on ADT experienced greater bother at loss of sexual function than patients not on ADT, suggesting that loss of libido when on ADT does not mitigate the psychological distress associated with loss of erections. In a multiple linear regression, patients' mood predicted their dyadic adjustment, such that worse mood was related to worse dyadic adjustment. However, more bother with patients' overall sexual function predicted lower relationship scores for the patients, while the patients' lack of sexual desire predicted lower dyadic adjustment for partners. Conclusions Both patients and partners are impacted by the prostate cancer treatment effects on patients' psychological and sexual function. Our data help clarify the way that prostate cancer treatments can affect relationships and that loss of libido on ADT does not attenuate distress about erectile dysfunction. Understanding these changes may help patients and partners maintain a co‐supportive relationship. Copyright © 2015 John Wiley & Sons, Ltd. Copyright © 2015 John Wiley & Sons, Ltd.