Ninth Circuit Court Modernization and Twelfth Circuit Court Creation Act of 2017 This bill divides the U.S. Court of Appeals for the Ninth Circuit into: (1) a new Ninth Circuit that consists of only California; and (2) a newly established Twelfth Circuit to be composed of Alaska, Arizona, Idaho, Montana, Nevada, Oregon, Washington, Guam, and Hawaii. The Twelfth Circuit must hold regular sessions in Las Vegas, Phoenix, Anchorage, Missoula, Portland, and Seattle. Each circuit judge of the former Ninth Circuit who is in regular active service and whose official duty station is currently in Alaska, Arizona, Idaho, Montana, Oregon, Washington, Guam, Hawaii, the Northern Mariana Islands, or Nevada shall be a circuit judge of the new Ninth Circuit. The President shall appoint, with the advice of the Senate, 17 circuit judges for the new Twelfth Circuit, selected from the states assigned to that circuit. Senior circuit judges of the former Ninth Circuit currently stationed in Alaska, Arizona, Idaho, Montana, Oregon, Washington, Guam, Hawaii, Nevada, or the Northern Mariana Islands may elect their circuit assignment.