The present technology relates to an image processing device, an image processing method, a program, and a projection system that facilitate the adjustment of the position and tilt of a camera. An image processing device according to one aspect of the present technology estimates, on the basis of a taken image taken by a camera installed at a predetermined tilt, the tilt of the camera, the camera being configured to photograph a projection surface of a screen on which a pattern image that is an image having a predetermined pattern has been projected from a projector, and performs control to display the taken image rotated depending on the tilt of the camera. The present technology is applicable to computers configured to control a plurality of projectors to project video.