Relative motions within the deforming Japanese Islands with respect to the Sea of Japan are determined using earthquake records over the last 414 years, slip rates on Quaternary faults, and angular change rates obtained from triangulation in the last century. We use a least squares inversion method to match the strain rates on the surface of the Earth with continuous spline functions to recover the velocity gradient tensors. The directions of the principal strain axes obtained from seismic, geological, and geodetic data are in general agreement with each other, with the maximum shortening axis oriented in a WNW direction. Principal strain rate magnitudes obtained from the geodetic data are about 3--5 times larger than the principal strain rates obtained from the seismic and geological data. Intraplate deformation in southwestern Japan determined from the seismic data accommodates a velocity of 5.5¿2 (1&sgr;) mm/yr in a direction parallel to the Nankai trough, which is about 25% of the plate motion velocity component parallel to the Nankai trough between the Philippine Sea and Eurasian plates. In northern Honshu, the velocity vectors from the seismic data are nearly parallel to the plate motion vector at the Japan trench, and the intraplate deformation accommodates about 6% of the total Pacific-Eurasian or North American plate motion velocity. The velocity vector at Kashima, northern Honshu, calculated from the geodetic data is about 9--12 mm/yr in a direction of N55¿W--N76¿W, which is about half of the very long baseline interferometry velocity calculated in the Eurasian reference frame, suggesting that the Sea of Japan may not be rigidly attached to the assumed rigid Eurasian plate. ¿ American Geophysical Union 1995