"Mexico will pay for the wall, believe me," Trump said in Phoenix. "They don't know it yet, but they're going to pay for the wall."
Speaking in Phoenix, the billionaire candidate also laid out his immigration plan that includes deporting immigrants with criminal records and cancelling President Barack Obama's executive orders protecting millions of undocumented immigrants.
Hours earlier, Trump met with Mexican President Enrique Pena Nieto and said the two did not discuss who would pay for the wall. But Pena Nieto contradicted Trump by tweeting that he told the Republican nominee in their meeting that Mexico would not fund the construction.
"At the start of the conversation with Donald Trump I made it clear that Mexico will not pay for the wall," Pena Nieto wrote on Twitter after Trump departed Mexico City for Phoenix.
In his speech, Trump laid out a grim, sweeping nationalist plan to dramatically slash illegal immigration, a main plank of his presidential campaign.
"Our enforcement priorities will include removing criminals, gang members, security threats, visa overstays, public charges -- that is those relying on public welfare or straining the safety net along with millions of recent illegal arrivals and overstays who have come here under this current corrupt administration."
Trump demanded an end to what he called "catch-and-release" programs along the country's southern border, enforcement of existing immigration law, and "zero tolerance for criminal aliens."
"We will be fair, just and compassionate to all," he stressed. "But our greatest compassion must be for our American citizens."