US President Donald Trump took out a new 30-year mortgage on a sprawling oceanfront house steps from his own Mar-a-Lago estate in Florida, and it is sitting empty on the rental market, according to financial disclosures made public on Thursday by the Office of Government Ethics.

Donald and Melania Trump with the President's sister Maryanne in 2005. Credit:AP

The $US18.5 million ($26.9 million) West Palm Beach mansion was purchased in May 2018 from one of Trump's older sisters and was secured with an $US11.2 million mortgage that has a 4.5 per cent interest rate, Florida public records show.

Trump disclosed the loan as a liability in financial disclosure forms filed this week.

A real estate website lists the eight-bedroom house, which is near the Mar-a-Lago beachfront estate and private club in Palm Beach, as available for rent for about $US81,000 a month.