Meanwhile, Spanish Prime Minister Mariano Rajoy will miss an European Union summit in order to deal with the referendum, a government source said.
The Spanish government has sent thousands of police officers to Catalonia in the run-up to Sunday's planned referendum.
Spanish authorities say the October 1 ballot on whether the Catalan region should secede from Spain is illegal. Trump said. "I'm just for a united Spain", he said, adding that if accurate polling were done in the region "you'd find out people of Catalonia love their country, they love Spain".
President Trump's decision to chime in reflects the largely anti-secessionist views across the West right now, with many nations fearing they'd face similar votes if Catalonia succeeded.
"I really think the people of Catalonia would stay with Spain", Trump said as he offered support to visiting Spanish Prime Minister Mariano Rajoy at the White House Tuesday.
In an order to police issued on Monday, the prosecutor's office said they would take the names of anyone participating in the vote and confiscate relevant documents.
The Spanish prime minister asked Catalan officials to return to "common sense" while Trump dubbed any attempts to separate from Spain as a foolish step.
The independence movement is widespread enough that it is unlikely to dissipate if the regional government fails to convert a "yes" vote into a split from Spain.
Kathleen Brooks, research director at City Index, said: "If the Catalonia referendum goes ahead we expect this to be a disruptive event for the euro, with the potential for a sharp decline of up to five per cent initially in the single currency if the separatists win". Spain's central government says such a vote would be illegal.
The officials said Catalonia lacked a proper election commission, ballot boxes, ballot papers, a transparent census and election material, and other material needed to hold the vote.
Many had not yet received information about where or when they would be working after the state-run postal service was told to stop all mail related to the vote, Marta Rovira said in a radio interview. He has promised to declare independence within 48 hours of a victory for the yes campaign.