Research has demonstrated that core affective valence and enjoyment experienced during physical activity are positively related to levels of physical activity. However, very few studies have tested mechanisms which explain these positive associations. Therefore, this study aims to determine whether intrinsic motivation mediated the association between valence and enjoyment during physical activity in daily life and levels of moderate-to-vigorous physical activity (MVPA). Data was collected using Ecological Momentary Assessment (EMA) from 182 young adults (age range 17-29). Participants completed 49 EMA prompts over 7 days. At each prompt, participant reported if they were physically active, and their momentary affective valence and enjoyment. At baseline, participant also reported their level of intrinsic motivation to be physically active and levels of MVPA. Mediation analysis was conducted using a two-stage modelling in MixWILD. In the first stage, a mixed-effects model including prompts that participants were active was run to estimate random intercepts for momentary valence and enjoyment during physically activity. In the second stage model, mediation was tested with the random intercept estimated in the first stage model as the independent variable, intrinsic motivation as the mediator, and MVPA as the outcome. Results showed that there was a significant indirect association between affective valence (indirect effect = 19.35, p = .044) and enjoyment (indirect effect = 21.80, p = .024) during physical activity on levels of MVPA through intrinsic motivation. These results highlight the importance of promoting valence and enjoyment during physical activity to promote intrinsic motivations to be active, and ultimately greater participation in MVPA.