The relationship between arthritis or repetitive stress injuries (RSIs) in thumbs and rapidly increasing hours of smartphone usage is not fully elucidated. We evaluated axial joint reaction forces (AJRFs) and thumb torques in 19 healthy subjects performing typical smartphone tasks, which included tapping, tap game, and swiping. We measured force and torque when a subject tapped or swiped the panel of the smartphone and analyzed the motions of each joint using surface markers and motion capture systems. We calculated AJRFs and torques on each thumb joint using inverse dynamics. The results were then compared with representative activities such as computer keyboard typing and handwriting. The mean AJRFs/torques at the thumb carpometacarpal joint (CMCJ) while tapping the smartphone and tap gaming were 12.5 N/95.5 N mm and 21.1 N/187.21 N mm, respectively. AJRFs and torques were significantly higher during tap gaming activities than during simple tapping subtasks (p = 0.003 and p < 0.001, respectively). Compared with those during computer keyboard typing, the mean AJRFs and torques at the CMCJ during smartphone tapping was 3 (p = 0.075) and 1.4 times (p = 0.680) larger, respectively. Considering the rapidly increasing dependency on smartphones in our daily lives, long-term exposure of the thumb to repetitive AJRFs and torques may lead to an acceleration of arthritis or aggravation of RSIs in thumbs.