This study aimed to explore the psychological determinants of smartphone multitasking. Smartphone multitasking comprises the following three different subtypes: multitasking with nonmedia activities, cross-media multitasking with nonsmartphone media, and single-device multitasking within the smartphone. The primary motivations for smartphone multitasking were first identified - efficiency, utility, and positive affect - and the ways in which they are associated with the three subtypes were examined; among the primary motivations, efficiency and positive affect predicted the degree of total smartphone-multitasking behavior. The personality traits that are pertinent to all of the primary motivations - need for cognition (NFC) and sensation seeking (SS) - were also investigated. Further analyses revealed that the motivations for and the extent of smartphone multitasking can vary as functions of a user's NFC and SS. In this study, NFC was not only a meaningful predictor of the cognitive needs that drive smartphone multitasking but also increased the likelihood of multitasking through its interaction with SS.