Fort Belknap Indian Community Water Rights Settlement Act of 2023 This bill modifies and ratifies a specified water rights settlement agreement entered into by the United States, Montana, and the Fort Belknap Indian Community (i.e., the Gros Ventre and Assiniboine Tribes). The bill requires the community's water rights to be held in trust for the benefit of the community and its allottees. The community must enact a tribal water code to regulate its water rights. Additionally, the bill authorizes the Department of the Interior and the Department of Agriculture (as applicable) to enter negotiations with Montana to exchange certain state lands for federal lands to be held in trust for the benefit of the community. The bill also establishes the Aaniiih Nakoda Settlement Trust Fund (and specified accounts) for purposes of carrying out this bill, establishes the Fort Belknap Indian Community Water Settlement Implementation Fund (and specified accounts) for purposes of carrying out this bill, and provides funding for specified accounts established by the bill.