Feingold, who was defeated by Johnson in 2010, announced his candidacy in a 90-second video, citing his battles against money in politics as a prevailing factor in the decision to run again. Feingold said he wants to "bring back to the U.S. Senate strong independence, bipartisanship and honesty."
"People tell me all the time that our politics in Washington are broken and that multi-millionaires, billionaires and big corporations are calling all the shots. They especially say this about the U.S. Senate, and it's hard not to agree," he said.
Feingold was unseated after 18 years in the Senate. Johnson, a businessman, took 52 percent of the vote to Feingold's 47 percent.
In 2013, Feingold was named as a U.S. envoy to Africa's Great Lakes Region and the Democratic Republic of Congo. He stepped down from the post in March.
The National Republican Senatorial Committee said Feingold is a "desperate career politician."
"After decades in politics, Feingold's ego still can't grasp that he was soundly defeated by Oshkosh job creator Ron Johnson in 2010," communications director Andrea Bozek said. "Wisconsin families rejected Feingold's broken promises and his liberal record once and they are going to do it again.