Mr Trump told reporters at a White House briefing that Ms Harris “was very disrespectful to Joe Biden” during the Democratic primaries, and said “it’s hard to pick somebody that’s that disrespectful.”
He also noted that Ms Harris ”did very poorly” when she ran for the nomination, she dropped out in December, adding: “That’s like a poll.”
Mr Trump is trying to paint Ms Harris as too-far left, saying she wants to raise taxes, slash funds for the military and ban fracking.
He also noted her tough questioning of now-Supreme Court Justice Brett Kavanaugh during his Senate confirmation hearing.
Mr Trump said: “She was my No. 1 draft pick and we’ll see how she works out.”
Mr Biden’s campaign announced his decision to pick Ms Harris as his running mate earlier on Tuesday.
The president last month said she would be “a fine choice.”
