There are lens aberrations such as pin cushion and barrel distortion, but this does not appear to be one of them.
Another possibility (especially if it occurs with more than one lens) is that there is a misalignment somewhere. Most likely with the focus screen and/or mirror. Have you tried setting your camera on a tripod and using a bubble level to make sure the camera is level? That will tell you if it's a mirror/focus screen alignment problem or a remote possibility of the sensor being out of alignment.