Trump said on social media late on Friday the payments would come from the Medicare Improvement Fund, which he said was given $US2 billion ($A2.9 billion) by Congress to improve the Medicare fee-for-service program.
The announcement is the latest in a series of financial promises the Republican leader has made to voters heading into the November midterm elections where control of Congress is at stake.
Trump's Republican Party currently has a narrow majority in both the Senate and the House of Representatives but his approval rating has been low over his handling of the high cost of living for Americans during the US-Israeli war on Iran that has spiked oil and gas prices.
The White House released some details of Trump's plan on its website, saying most eligible seniors will receive the payment in the form of a direct deposit of $US90 in early October.
Those without direct deposit will receive a cheque, also sent in early October, to the mailing address they have registered with Medicare, the White House said.
In September, Trump proposed sending $US5,000 ($A7,195) cheques to all adult citizens if the Republicans keep control of the House and Senate in the midterms, though analysts, economists and even some Republicans in Congress reacted with skepticism to the plan.
The Trump administration is also separately sending $US500 ($A720) payments to nearly one million Americans in a move that strategists say could potentially influence some election races.
The White House says those payments were a refund for what it describes as excessive fees charged to people who purchased health insurance through the Affordable Care Act.
