<p>This study investigated how structured activity and the presence of another person jointly shape the experience of flow and its physiological correlates. Flow is typically described as a multidimensional state, yet little is known about how social presence alters its occurrence and how subjective reports align with physiological indices. To address this gap, twenty-four healthy young adults completed four within-subject conditions that systematically varied in activity (present vs. absent) and social context (another person present vs. absent). Flow was assessed using the Flow State Scale for Occupational Tasks (FSS-OT), and perceived stress with the Perceived Stress Scale (PSS). Autonomic regulation was indexed by three heart rate variability (HRV) measures: the cardiac vagal index (CVI) and the cardiac sympathetic index (CSI), derived from Lorenz plot geometry, and the root mean square of successive differences (RMSSD), reflecting vagal tone. FSS-OT scores were significantly higher in both activity conditions compared with the “with another person/without activity” condition, whereas PSS scores showed no significant condition differences. CVI increased in the “with another person/without activity” condition, suggesting adaptive vagal modulation under neutral co-presence, whereas RMSSD remained stable—indicating distinct autonomic patterns across indices. No significant correlations were found between changes in CVI and flow, indicating that physiological regulation and subjective immersion do not necessarily covary. These findings extend psychological models of flow by showing that physiological regulation and subjective immersion can coexist within neutral co-presence. Overall, this study deepens understanding of how individual and contextual factors jointly contribute to maintaining flow experience.</p>