Intimate relationship partners dynamically covary in their affective states. One mechanism through which intimate relationship partners experience and shape each other's affective states is affect contagion, that is, the spread of affective states from one person to another. Most studies on affect contagion in daily life focused on younger couples. Also, the degree to which social-cognitive processes are involved in everyday affect contagion remains unclear. Intimate relationships are one of the most important social contexts in older adults' daily lives. Expanding on previous research, we focused on affect contagion in older couples, and examined whether processes of affect contagion were mediated by perceptions of partner affect, that is, how individuals thought their partners felt at previous moments. We used data from an experience sampling study with 152 older heterosexual couples (304 participants; 65 + years old) who reported on their positive and negative affect, perceptions of their partner's positive and negative affect, and presence or absence of partners 6 times a day for 14 days. Dyadic multilevel mediation models were used to evaluate our hypotheses. We observed consistent evidence that processes of positive affect contagion between partners were mediated by perceptions of partner's affective states. Negative affect contagion was directed from men to women, but not vice versa, and mediated by perceptions of partner's affective states. Partner presence was unrelated to processes of affect contagion. Our findings support the notion that (perceptions of) close others' emotions shape our own feelings and identify partner perceptions as a mechanism of affect contagion. (PsycInfo Database Record (c) 2022 APA, all rights reserved).