Blake Gibbins, a 26-year-old from Lafayette, Colorado, was arrested at Mineral City Mill and Grill restaurant in Fort Dodge, central Iowa.
Police said King was having a group lunch at 12:30 p.m. on Friday when Gibbins approached their table.
Gibbins asked King to confirm his identity and then threw a glass of water on him, splashing King and another man.
Gibbins, who runs a YouTube channel called Not Your Orphan about adoption rights from the perspective of adoptees, declined to speak to BuzzFeed News about why he threw the water at King.
"I am sorry that I am not able to respond at this time (as you can likely understand, there is a lot going on at this exact time)," said Gibbins via Facebook message.
The Colorado resident was charged with simple assault and disorderly conduct, both misdemeanors.
His most recent video called for Bernie Sanders to host a 2020 town hall with adoption advocates about adoptee rights, and to speak about the history of eugenics and human rights abuses in the adoption industry.
He posted the video with the tags "democrats," "progressive," "feminist," "berniesanders," and "aoc," although he noted the video was not an endorsement of Sanders.
King won reelection in the 2018 midterms, despite renewed pressure regarding his support of white supremacist views.
The congressman, who has represented a sprawling northwest Iowa district since 2003, has repeatedly made racist statements, speaking often about his fear of the US losing its white Christian majority to immigrants and people of color, with little outcry from his Republican colleagues.
“Diversity is not our strength,” he tweeted in December 2017, while quoting Hungary’s far-right prime minister as saying, "Mixing cultures will not lead to a higher quality of life but a lower one."
In an interview last August with Unzensuriert, a far-right Austrian site founded by a former Nazi SS officer and aligned with Austria’s far-right Freedom Party, King spoke about his white nationalist beliefs, including the “Great Replacement” conspiracy theory that Muslims and immigrants of color are replacing white people.