Background and aims: Problematic Smartphone Use (PSU) has become a major public health issue, with stress identified as a key factor. Pathological technology use is often linked to dysfunctional reward processing, which is characterized by hyperactivity during reward anticipation and hypoactivity during reward receipt, both closely tied to emotion regulation. This study aimed to investigate the association between PSU and event-related potentials (ERP) linked to reward anticipation and feedback processing, while elucidating the role of reward processing dysfunction in the escalation of daily life stress into PSU through ecological momentary assessment. Methods: We recorded the ERPs of 44 PSU participants and 50 HC participants during the monetary incentive delay task. Meanwhile, we assessed the momentary stress, PSU levels, and screen time of these participants three times a day for 14 days. Results: ERP results showed that the PSU group, compared to the HC group, had significantly larger P3 amplitude (but not N2 amplitude) during reward anticipation (cue-P3: n2 5 0.066, p 5 0.012; cue-N2: n2 5 0.004, p 5 0.567). In contrast, during feedback, their amplitudes were reduced in both RewP and fb-P3 components (RewP: n2 5 0.092, p 5 0.003; fb-P3: n2 5 0.043, p 5 0.048). These findings indicate that PSU is linked to heightened neural activity during reward anticipation but reduced responsiveness during feedback, indicating potential dysfunction in reward processing. Ecological momentary assessment linked momentary stress to increased PSU (ß 5 0.17, HPD 95% CI [0.129, 0.218]) and screen time (ß 5 0.18, HPD 95% CI [0.135, 0.227]). Importantly, RewP amplitude moderated these associations, with blunted RewP responses amplifying stress-related increases in both PSU (ß 5 0.19, HPD 95% CI [0.352, 0.036]) and screen time (ß 5 0.20, HPD 95% CI [0.394, 0.003]). Conclusion: These findings indicate that reward-related ERPs may serve as potential neural markers for identifying PSU, while dysfunctional reward processing may exacerbate stress-related PSU behaviors. This work provides novel insights for developing prevention and intervention strategies in digital addiction  Kulcsszavak: problematic smartphone use, stress, reward processing, event-related potential, ecological momentary assessment