We introduce GripSense, a system that leverages mobile device touchscreens and their built-in inertial sensors and vibration motor to infer hand postures including one- or two-handed interaction, use of thumb or index finger, or use on a table. GripSense also senses the amount of pres-sure a user exerts on the touchscreen despite a lack of direct pressure sensors by inferring from gyroscope readings when the vibration motor is "pulsed." In a controlled study with 10 participants, GripSense accurately differentiated device usage on a table vs. in hand with 99.67% accuracy and when in hand, it inferred hand postures with 84.26% accuracy. In addition, GripSense distinguished three levels of pressure with 95.1% accuracy. A usability analysis of GripSense was conducted in three custom applications and showed that pressure input and hand-posture sensing can be useful in a number of scenarios.