Madison County is a county located in the U.S. state of Idaho. As of the 2020 census, the population was 52,913. The county seat and largest city is Rexburg. Madison County is part of the Rexburg, Idaho micropolitan area, which is also included in the Idaho Falls-Rexburg-Blackfoot Combined Statistical Area.

The area was originally settled by members of the Church of Jesus Christ of Latter-day Saints. Before February 1913, the county was part of neighboring Fremont County. The newly established county was named for American president James Madison. Brigham Young University–Idaho, formerly Ricks College (named after early local LDS settler Thomas Edwin Ricks) is located in Madison County. Madison County was declared a national disaster area after the flood of June 5, 1976.

Madison County is the owner of the healthcare system in the region. However, it is contracted out to Madison Memorial. Madison Memorial began in 1951 when the doctors at that time decided it was time for the community to have a hospital. They then closed their practices and collaborated with the community to build Madison Memorial, a non-profit healthcare system. Since that time, Madison Memorial has continued to grow, promote population health for the region, and provide professional healthcare services for the region with over 800 employees. The region served includes the following counties Jefferson, Madison, Fremont, Teton, Clark, and Lemhi. Madison Memorial is the nearest hospital to Yellowstone National Park.

Similar to other Idaho counties, an elected three-member county commission heads the county government. Other elected officials include clerk, treasurer, sheriff, assessor, coroner, and prosecutor.

With a strongly conservative population, Madison County is one of the most staunchly Republican counties in the United States. Since 1968 no Republican presidential candidate has failed to carry the county with less than 56 percent of the vote, and no Democratic presidential nominee has cracked 23 percent thereof. In that same period Republican presidential candidates polled more than 90 percent of the county's vote on three occasions, Ronald Reagan in 1984, George W. Bush in 2004, and Mitt Romney in 2012. John McCain came close to this level in 2008, drawing 85 percent of the vote. In 2016, Donald Trump won the county, but performed far worse in it than Republicans typically do: he received just 57 percent of the vote, while Romney had received over 93 percent of the vote there just four years earlier. However, this is attributed to the county giving Evan McMullin almost thirty percent of the vote in 2016, which was his best performance of any county in the entire country that year.

In 2020, Trump won 79% of the vote, 22 points up from 2016. However this was still a lower vote share than those achieved by Republican candidates George W Bush in 2000 and 2004, John McCain in 2008 and Mitt Romney in 2012.

Joe Biden won 15.6%, up 7.9% from Hillary Clinton's vote share in 2016. Biden's vote share was the highest for a Democrat in a presidential race in this county since 1996. It was also one of just four times since Lyndon Johnson's 1964 landslide that a Democrat exceeded 15% (the others being the aforementioned 1996, as well as Jimmy Carter in 1976 and Hubert Humphrey in 1968).

At the state level Madison County is located in Legislative District 34, which currently has an all-Republican delegation in the Idaho Legislature.