Harvey has brought the heaviest rainfall in US history, soaking Texas with more than 52 inches since it first made landfall last Friday.
At least 38 people have been killed as a result of the devastating floods and that number was climbing as water receded.
"He will pledge, proudly, a million dollars of his own personal money to help the people of both Texas and Louisiana," White House Press Secretary Sarah Sanders said.
"I had a chance to speak directly with the president earlier, and I'm happy to tell you that he would like to join in the efforts that a lot of the people that we've seen across this country do, and he's pledging a million dollars of personal money to the fund," she said.
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Trump travelled to Texas on Tuesday for a first-hand assessment of the situation on the ground.
He would return to the Houston area over the weekend, she said.
"It looks like around about 100,000 affected homes. That's a big number. We're going to have 100,000 affected homes, all with different degrees of insurance; some with flood insurance, some under insured, some uninsured," he said.
"We'll make that request shortly. We'll make that request based on the information that we have now. What we'll do then is come back later for a second supplemental request when we have additional information that would make a more informed total for Congress to consider," he said.
Accuweather estimated Harvey's cost at USD 160 billion, making it thecostliest natural disaster in US history.
AccuWeather president Joel Myers called Harvey a '1000 year storm' and said parts of Houston will be uninhabitable for weeks or months.