Social media (SM) is widely used among adolescents, making the relationship between SM and affect in this population important to understand. However, previous research has largely used methods that do not capture the dynamic and variable nature of SM or measure the consistency of effects across global affect and specific emotions. The current study examined the momentary relationship between positive and negative SM experiences and global positive affect (PA) and negative affect (NA), as well as specific affective states of PA and NA. Adolescents (N = 62; 14-17 years old) were recruited from SM platforms to participate in an eight-week ecological momentary assessment study. Participants reported three times per day on PA and NA and positive and negative SM experiences. Random intercept and random slope models were used to examine within-person associations between momentary positive and negative SM experiences and affect after controlling for between-person differences. At prompts when participants reported greater-than-usual positive SM experiences, they reported higher global PA and lower global NA. At prompts when participants reported greater than their average negative SM experiences, they endorsed lower global PA and higher global NA. These results were consistent across same-valence specific affective states. The current study highlights proximal associations between SM experiences and affect, highlighting potential avenues of short-term risk and protection.