Over the past few years, a growing amount of research has considered the links between personality and overuse (pathological use) of the Internet. Given the partial overlap between problematic Internet and smartphone use (PIU and PSU, respectively), the present study seeks to investigate whether the same personality traits can be linked to overuse of both platforms. A total of 612 participants (177 males/435 females, mostly students) completed questionnaires assessing both PIU and PSU, and the NEO Five Factor Inventory (NEO-FFI) to measure the Five-Factor Model of Personality and the Self-Directedness scale of the Temperament and Character Inventory. Our results indicate the existence of a common personality structure linked to both PIU and PSU. Interestingly, the associations between personality and PIU were higher than those concerning PSU. Low Self-Directedness appears to be the best predictor of overuse on both digital platforms. Therefore, lower willpower anchored in the personality trait Self- Directedness may reflect the core of digital additive tendencies.