Mr Joyce’s office said in a statement the Nationals leader had experienced “mild symptoms” before deciding to get tested.
“Remaining members of the delegation have tested negative,” the statement read.
“Mr Joyce will remain in isolation until further advice.”
He was on a 10-day overseas trip, stopping in the UK before visiting the US.
He met with UK Deputy Prime Minister Dominic Raab on Tuesday and was set to meet with Texas Republican Ted Cruz and other US politicians during his visit.
The diagnosis comes at the end of a tumultuous year for Mr Joyce, who was reappointed to leader of the Nationals in June.
He had previously held the position from February 2016 to February 2018, when he resigned after revealing he was expecting a child with a former staffer.