We propose TiltChair, an actuated ofce chair that physically manipulates the user's posture by actively inclining the chair's seat to address problems associated with prolonged sitting. The system controls the inclination angle and motion speed with the aim of achieving manipulative but unobtrusive posture guidance. To demonstrate its potential, we frst built a prototype of TiltChair with a seat that could be tilted by pneumatic control. We then investigated the efects of the seat's inclination angle and motions on task performance and overall sitting experience through two experiments. The results show that the inclination angle mainly afects the difculty of maintaining one's posture, while the motion speed afected the conspicuousness and subjective acceptability of the motion. However, these seating conditions did not afect objective task performance. Based on these results, we propose a design space for facilitating efective seat-inclination behavior using the three dimensions of angle, speed, and continuity. Furthermore, we discuss promising applications.