In a first embodiment of a camshaft phaser, the rotor is split along an equatorial plane thereof, defining first and second rotor portions. A resilient seal is compressively disposed between the rotor portions to prevent leakage therebetween and to urge the rotor vanes against the cover plate and sprocket surfaces to prevent leakage around the vanes. Preferably, the seal includes radial seal elements to seal against the stator wall. Preferably, pressurized hydraulic fluid can enter between the first and second rotor portions and thereby assist the resilient seal in urging the rotor portions apart. In a second embodiment, the rotor is not split, and a resilient seal is provided on at least one axial face of the rotor vanes to seal the rotor vanes against the cover plate and sprocket surfaces.