We determined principal horizontal stress orientations as of 1999 at sites in the source area of the 2011 M-w 9.0 Tohoku-Oki earthquake, on the deep-sea terrace of the Japan Trench. The maximum principal horizontal stress orientation at site 1151 of Ocean Drilling Program Leg 186, located in an aseismic zone, was east-southeast, parallel with the plate convergence direction; and the stress orientation at site 1150 of Leg 186, in a seismically active zone, was south-southeast, suggestive of a local stress feature due to the influence of seismic activity. Aftershocks with normal faulting mechanisms, observed in the hanging wall of the plate interface that ruptured during the Tohoku-Oki earthquake, indicate a normal faulting postseismic stress regime, which clearly differs from the stress state prior to the earthquake. Consequently, the stress state may have changed from a reverse to a normal faulting stress regime during the Tohoku-Oki earthquake sequence. Citation: Lin, W., S. Saito, Y. Sanada, Y. Yamamoto, Y. Hashimoto, and T. Kanamatsu (2011), Principal horizontal stress orientations prior to the 2011 M-w 9.0 Tohoku-Oki, Japan, earthquake in its source area, Geophys. Res. Lett., 38, L00G10, doi:10.1029/2011GL049097.