Since becoming speaker of the House in October, Ryan has come under attack from more conservative members of his party for pushing a compromise budget deal and for being unable to implement conservative priorities.
Nehlen said that he had worked hard for Ryan but has become increasingly frustrated with his agenda.
"I worked behind the scenes for Paul Ryan, and that’s where this (motivation to run) all came from," Nehlen told the website.
But in the end, Nehlen says, Ryan let him down.
Nehlen serves as senior vice president of operations in Lake Geneva for Neptune Benson, a company that leads the industry in water filtration and disinfection technologies.
Nehlen also runs his own small business, Blue Skies Global LLC.
Two Democrats and a Libertarian also have announced they will challenge Ryan.
Ryan A. Solen of Mount Pleasant, a military veteran, is an information security analyst at SC Johnson and Son in Racine.
Tom Breu of Janesville, also a Democrat, has been in the running for months.
Jason Lebeck, a Janesville Libertarian, said in a news release he wants to restore the national government to its constitutionally mandated role as defender of citizens’ rights.
The partisan primary election is Aug. 9. The general election is Nov. 8.
It will be an uphill climb to unseat Ryan, who first was elected in 1998.
As speaker of the House, Ryan is the Republican Convention chairman and could oversee a tumultuous nominating process if none of the three remaining Republican candidates -- Donald Trump, Ted Cruz and John Kasich -- arrives at the convention with enough delegates to secure the nomination.
Few in the Republican Party establishment are supporting Trump’s unconventional candidacy and believe there could be an open convention.
Rules for delegates are different for each state, but in most instances, delegates are free to vote for whomever they choose after one or two ballots without a nominee gaining a majority vote.
In that case, there’s been an increasingly vocal call to draft Ryan as the Republican presidential nominee. Ryan ran four years ago as Mitt Romney’s running mate in their failed bid.
Ryan repeatedly has said he is not interested in the Republican nomination, but his supporters point out that he protested similarly before eventually agreeing to run for the House speakership.