In this work we report on a simple inertial microfluidic device that achieves continuous multi-particle separation using the principle of Dean-coupled inertial migration in spiral microchannels. The dominant inertial forces coupled with the Dean rotational force due to the curvilinear microchannel geometry cause particles to occupy a single equilibrium position near the inner microchannel wall. The position at which particles equilibrate is dependent on the ratio of the inertial lift to Dean drag forces. Using this concept, we demonstrate, for the first time, a spiral lab-on-a-chip (LOC) for size-dependant focusing of particles at distinct equilibrium positions across the microchannel cross-section from a multi-particle mixture. The individual particle streams can be collected with an appropriately designed outlet system. To demonstrate this principle, a 5-loop Archimedean spiral microchannel with a fixed width of 500 µm and a height of 130 µm was used to simultaneously and continuously separate 10 µm, 15 µm, and 20 µm polystyrene particles. The device exhibited 90% separation efficiency. The versatility of the device was demonstrated by separating neuroblastoma and glioma cells with 80% efficiency and high relative viability (>90%). The achieved throughput of ∼1 million cells/min is substantially higher than the sorting rates reported by other microscale sorting methods and is comparable to the rates obtained with commercial macroscale flow cytomerty techniques. The simple planar structure and high throughput offered by this passive microfluidic approach make it attractive for LOC devices in biomedical and environmental applications.