Johnson's victory keeps the Senate seat red in a critical swing state.
With 94 percent of precincts reporting as of 1 p.m. ET, Johnson had won 50.5 percent of the vote, compared to Barnes' 49.3 percent. The victory for Johnson marks a narrow win for the incumbent senator, and he is now poised to serve his third term in Congress.
"There is no path mathematically for Lt. Gov. Barnes to overcome his 27,374 vote deficit. This race is over," Johnson said in an email statement.
Barnes had not yet conceded as of Wednesday, and a spokesperson for his campaign said in a statement, "We always knew this race would be incredibly close. No matter what anyone says, we are committed to making sure every vote is counted. We will wait and see what the Wisconsin voters have decided after all their voices are heard."
During an interview with WISN-AM, per AP, Johnson said he was surprised the race was "this close."
The close outcome was representative of a Senate seat that Democrats had pushed hard to flip for Barnes, with former President Barack Obama sharply criticizing Johnson in a now-viral moment from the campaign. However, the Democratic effort came up short amidst Johnson's significant backing from Republican leaders, including former President Donald Trump.
Johnson's victory could play a key role in determining which party retains control of the Senate.