Tongue River Dam Authorization Act of 1990 - Authorizes the Secretary of the Interior to plan, design, and construct improvements to the Tongue River Dam in Montana and to provide for the operation and maintenance of the project. Requires Montana to provide a portion of the project's costs. Requires a portion of any increased capacity of the Tongue River Dam to be provided to the Northern Cheyenne Tribe as part of any water rights settlement reached between the Tribe and Montana.

Authorizes the Secretary to assist the Tribe in developing acreage for agricultural purposes in accordance with the Missouri River Basin Program of the Flood Control Act of 1944. Requires land developed for such purposes to be located within the Northern Cheyenne Reservation.

Requires the U.S. Fish and Wildlife Service, the Montana Department of Fish, Wildlife, and Parks, the Montana Department of Natural Resources, and the U.S. Bureau of Reclamation to identify project features for the enhancement of fish and wildlife habitats for development by the Secretary.

Authorizes appropriations for the Tongue River project.