Satisfaction and frustration of the basic psychological need for relatedness have been postulated to play a vital role for affective well-being. Yet, this prediction has not been thoroughly tested in schoolchildren's everyday lives. In this work, we examined the association between relatedness satisfaction and frustration at school on daily and average positive affect (PA) and negative affect (NA) using ambulatory assessment in three intensive longitudinal studies with children aged 9-12 (total N = 317). In Study 1, 4th to 6th graders reported their PA and NA 2 times daily and their relatedness satisfaction and frustration once a day for 2 weeks. In Study 2 and Study 3, 4th graders and 5th graders, respectively, reported their PA and NA 4 times daily and their relatedness satisfaction and frustration once a day for 4 weeks. Across the 3 studies, relatedness satisfaction and frustration were psychometrically separable and exhibited differential effects such that relatedness satisfaction was significantly associated primarily with PA, and relatedness frustration was significantly associated only with NA at between- and within-person levels. Explaining interindividual differences suggested that the association between daily relatedness and affective well-being was weaker for generally highly integrated children and stronger for usually rather excluded children. (PsycInfo Database Record (c) 2020 APA, all rights reserved).