Buzz Aldrin is suing two of his children and a business manager, accusing them of fraud, exploitation of the elderly and slander by suggesting he has dementia.
The 88-year-old former astronaut filed the lawsuit in a Florida state court a week after his children, Andrew and Janice, filed a petition claiming their father was suffering from memory loss, delusions, paranoia and confusion.
They asked for the court to name them his legal guardians, saying Mr Aldrin was associating with new friends who were trying to alienate him from his family and that he had been spending his assets at "an alarming rate".
Court-appointed mental health experts planned to evaluate Mr Aldrin in Florida this week.
In April, Mr Aldrin underwent his own evaluation conducted by a geriatric psychiatrist, who said Mr Aldrin scored "superior to normal" for his age on tests.
"I also believe that he is perfectly capable of providing for his physical health needs, food, clothing and shelter, and is substantially able to manage his finances and resist fraud and undue influence," said Dr James Spar in a letter to Mr Aldrin's lawyer.
In the lawsuit, the Apollo 11 moonwalker asked a judge to remove Andrew Aldrin from control of his financial affairs, social media accounts and several nonprofit and business enterprises.
Complaint
Mr Aldrin said in the complaint that despite revoking the power of attorney he had given his son, Andrew Aldrin continued making financial decisions for him.
In the lawsuit, he accuses his daughter Janice Aldrin of not acting in his financial interests and conspiracy.
Mr Aldrin also accuses his former manager, Christina Korp, of fraud, exploitation of the elderly and unjust enrichment.
Mr Aldrin's oldest son, James, is not involved in the legal fight.
In a statement, Andrew and Janice Aldrin said they were saddened by the "unjustifiable" lawsuit.
"If nothing else, our family is resilient and our ability to work together to solve problems and accomplish great things is strong," the Aldrin children said.
"We love and respect our father very much and remain hopeful that we can rise above this situation and recover the strong relationship that built this foundation in the first place."
They said they would have no further comment.
Mr Aldrin was a member of the Apollo 11 crew which landed the first two humans on the moon.
He joined Neil Armstrong on the lunar surface in July 1969.
Earlier this month, Mr Aldrin was at the White House for Donald Trump's announcement directing the Pentagon to create the "Space Force" as a new military branch.
