Difficulty regulating substance use is a core feature of addiction that can manifest as unplanned use. This study sought to identify internal and situational influences on unplanned marijuana use among youth ages 15 to 24 years (N = 85; 48% female; 27% age <18 years). Additionally, we disentangled person-level associations from within-person day-to-day influences. Ecological momentary assessment methods captured affective (positive: energized, excited, sociable, happy, relaxed; negative: bored, tense, sad, stressed) and situational factors in real-world settings during a 1-week monitoring period. Participants reported no plan to use on 51% of days (269/527), and youth ultimately used marijuana on 35% of these unplanned days. At the day level, on days when youth spent more time in the presence of marijuana-related cues than they typically do, they used more grams on planned days and less on unplanned days. Regardless of use plans, youth were more likely to use on days when they spent more time with using friends and if they reported greater availability of marijuana in general across the monitoring period. At the person level, youth who generally reported higher positive affect, relative to other participants, used more on planned days and less on unplanned days. Regardless of use plans, youth who generally reported greater craving and time in the presence of marijuana-related cues used more grams, whereas youth who generally reported greater negative affect used less. Together, findings revealed several factors, with clear clinical relevance, which may explain why some youth struggle to control their marijuana use. (PsycInfo Database Record (c) 2020 APA, all rights reserved).