The New England MP and former deputy prime minister announced the move to the far-right party on Monday.
He will sit in the House of Representatives for the rest of the parliamentary term as a One Nation MP, before leading the party's Senate ticket for NSW.
Mr Joyce turned his back on the Nationals after three decades with the rural party. He said he had given serious consideration to the move.
"I am firmly of the view, which I have considered over a long period of time, that the best choice before me is to stand for One Nation as a Senator for NSW," Mr Joyce said in a statement on social media.
"I will let the voters be the ultimate arbiter of that decision."
Mr Joyce announced his defection from the Nationals on the last sitting day of the year in November, citing a breakdown in the relationship with party leadership and being relegated to the back bench after the May election.
At the time, he declined to say what his next political move would be.
Mr Joyce's widely flagged decision to quit the Nationals came as no surprise. Speculation increased about the move to One Nation after Mr Joyce was seen dining with Senator Hanson at Parliament House over a wagyu steak cooked on a sandwich press.
Senator Hanson said she was pleased the MP would be joining the party.
"Mr Joyce strengthens One Nation's position in parliament just as many Australians are strengthening our position in the polls," she said in a statement.
"I have always been very straightforward about asking Mr Joyce to join our team, and on making it clear this was always his decision."
Senator Hanson said the addition of Mr Joyce to the One Nation ranks would give the party a boost ahead of the next federal election, due to be held by May 2028.
"We're going to be the A-team, I tell you what. Going into the next election, be prepared," she told a Tamworth radio station.
The party leader visited Tamworth on Monday to announce the move.
Nationals leader David Littleproud said the decision by Mr Joyce to leave the party was disappointing.
“One Nation is a party of protest, not a party of government,” he said in a statement.
“Barnaby’s decision breaks the contract he made with the people of New England at the 2025 federal election.
“I have never had a personal issue or problem with Barnaby Joyce. This issue is about Barnaby wanting to be the leader of a party.”
Recent polling shows the number of people planning to put One Nation first on their ballot paper is at an all-time high, with surveys putting support for the party between 14 and 18 per cent.
A recent Resolve poll said 29 per cent of voters would be more likely to support the party if it were led by Mr Joyce.
Mr Joyce will be the first member of One Nation to sit in the lower house since Senator Hanson herself in the late 1990s.
– With AAP
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