This brief presents the design of a low-power 3-lead electrocardiogram (ECG)-on-chip with integrated real-time QRS detection and lossless data compression for wearable wireless ECG sensors. Data compression and QRS detection can reduce the sensor power by up to 2-5 times. A joint QRS detection and lossless data compression circuit allows computational resources to be shared among multiple functions, thus lowering the overall system power. The proposed technique achieves an average compression ratio of 2.15 times on standard test data. The QRS detector achieves a sensitivity (Se) of 99.58% and positive productivity (+P) of 99.57% @ 256 Hz when tested with the MIT/BIH database. Implemented in 0.35 μm process, the circuit consumes 0.96 μW @ 2.4 V with a core area of 1.56 mm² for two-channel ECG compression and QRS detection. Small size and ultralow-power consumption makes the chip suitable for usage in wearable/ambulatory ECG sensors.