Mark Allen Callahan (born May 11, 1977) is an American information technology consultant and perennial candidate. He was the Republican nominee in the 2016 United States Senate election in Oregon.

Callahan is known as a perennial candidate, having frequently sought office since 2009 in both Oregon and most recently in Texas.

Callahan sought appointment to a seat in the Oregon Senate in 2009 as a Democrat.

In 2010, he ran for the Lane County Commission. Later in 2010, Callahan ran for the Oregon House of Representatives, unsuccessfully seeking the nomination of the Independent Party of Oregon before receiving the nomination of the Pacific Green Party, in what he later said was an attempt to take away votes from the Democratic incumbent, Nancy Nathanson, in order to help the Republican nominee win. The attempt was unsuccessful.

In 2011, Callahan unsuccessfully sought a school board seat in Eugene. His then-wife, Sherry, also unsuccessfully ran for a different seat on the board in the same election.

In 2012, Callahan ran for President of the United States as a Republican. He filed to run in both the New Hampshire and Arizona primaries.

Later in 2012, Callahan won the Republican nomination without opposition for the same Oregon House of Representatives seat he had sought in 2010, losing again to Nancy Nathanson in the general election.

In 2014, Callahan sought the Republican nomination for the U.S. Senate, receiving nearly 7% of the vote and coming in third place, behind Jason Conger and eventual nominee Monica Wehby. During this campaign, Callahan was featured on Fox News after an argument with Nigel Jaquiss, a reporter, resulted in him being kicked out of a Willamette Week editorial interview.