First Interstate BancSystem, Inc., is a financial holding company both the company and its primary banking subsidiary First Interstate Bank. Both are based in Billings, Montana.

The bank originated in 1968 as a single bank in Sheridan, Wyoming, and incorporated in Montana in 1971. One of the banks that is now part of the system was the First National Bank of Casper which traces its roots to the C.H. King and Company Bankers founded in 1889 by Charles Henry King, grandfather of President Gerald Ford.

In 1984, the company entered into a franchise agreement with First Interstate Bancorp, a multi-state bank holding company headquartered in Los Angeles, California, to use the First Interstate Bank name and logo. In 1996, when First Interstate Bancorp was split up, selling its Midwestern USA branches to St. Louis-based Boatmen's Bancshares, which set the stage for it remaining west coast operations to be purchased by and merged into Wells Fargo. The Montana organization successfully negotiated to retain the well known First Interstate name and logo. As of 2006, the bank had expanded to 49 branches with 130 ATMs in Montana and Wyoming. In November 2016, it acquired Cascade Bancorp of Oregon for $589 million. At that time, First Interstate had $7.3 billion in deposits and $9 billion in assets.

First Interstate Bank is a fifty-percent owner and primary tenant of the First Interstate Center, a 20-story building in Billings, Montana. Completed in 1985 the 272 foot (83 m) structure is the tallest building in the state of Montana.