Boundary County is the northernmost county of the U.S. state of Idaho. As of the 2020 census, the population was 12,056. The county seat and largest city is Bonners Ferry.

Boundary County was created by the Idaho Legislature on January 23, 1915. It is so named because it borders Canada, and is therefore the only county in Idaho with an international border. It is also only one of three counties in the United States that borders two states and a foreign country, the others being Coös County, New Hampshire, and (via a water rather than land border) Erie County, Pennsylvania; these three counties additionally make up the entirety of the international border of their respective states, the only three counties nationwide to do so.

Boundary County was formed on January 23, 1915, from Bonner County. It was named Boundary County because it lies on the border of Canada, Washington and Montana.

Boundary County has seven election precincts: Bonners Ferry, Copeland, Kootenai, Moyie, Naples, North Bonners Ferry, and Valley View. All contain part of Bonners Ferry City except Copeland, Moyie, and Naples precincts. Moyie Springs was incorporated in 1947.

Settlement of the area started with the establishment of Bonners Ferry on the Kootenai River in 1864. Settlement was limited to the ferry operation until about 1890. The town of Bonners Ferry was established in 1893. At that point settlement was still sparse with small ranching and mining operations, but an expanding timber economy. By 1900, other areas started to develop with the Boulder (now Kootenai), Boundary (now Copeland), and Naples precincts first listed in the U.S. Census of that year. The Moyie precinct first appeared in the 1910 census.

In 1980, convicted spy Christopher John Boyce found refuge in Boundary County, for a few months, after his escape from the Lompoc Federal Correctional Complex. He stayed at the home of Gloria Ann White. Boyce sustained himself during his stay with a series of bank robberies in the surrounding area, allegedly with technical assistance from White.

In 1992, Boundary County was the scene of the Ruby Ridge siege by 350–400 armed federal agents against Randy Weaver and his family.

In April 2024, the city council of Bonners Ferry, the county seat of Boundary County, unanimously passed a resolution to continue flying the Canadian flag at its visitor center, despite state restrictions that threatened to remove all foreign flags from government properties. The resolution was seen as a symbolic gesture honoring the region's historic and economic ties with neighboring British Columbia, especially amid growing tensions over state-level efforts to restrict certain displays of international symbols on public grounds.