Although a majority of adults live with a close relationship partner, little is known about whether and how partners’ momentary affect and physiology covary, or “coregulate.” This study used a dyadic multilevel modeling approach to explore the coregulation of spouses’ mood states and cortisol levels in 30 married couples who sampled saliva and reported on mood states 4 times per day for 3 days. For both husbands and wives, own cortisol level was positively associated with partner’s cortisol level, even after sampling time was controlled. For wives, marital satisfaction weakened the strength of this effect. Partner’s negative mood was positively associated with own negative mood for both husbands and wives. Marital satisfaction fully moderated this effect, reducing the strength of the association between one’s own and one’s partner’s negative mood states. Spouses’ positive moods were not correlated. As expected, within-couple coregulation coefficients were stronger when mood and cortisol were sampled in the early morning and evening, when spouses were together at home, than during the workday. The results suggest that spouses’ fluctuations in negative mood and cortisol levels are linked over several days and that marital satisfaction may buffer spouses from their partners’ negative mood or stress state.(PsycINFO Database Record (c) 2016 APA, all rights reserved)