A Manhattan prosecutor says a feeble Brooke Astor signed a letter giving her son $5 million on the same day that she was too confused to know she was in her own house.

Assistant District Attorney Joel Seidemann said Friday that the incident in 2003 was further proof that Astor’s son took advantage of Astor’s mental decline to plunder her $198 million estate.

Seideman is finishing closing arguments Friday in the trial of Astor’s son, Anthony Marshall and attorney Francis Morrissey.

Prosecutors say Astor — who died in 2007 at age 105 — wasn’t mentally competent to sign documents that increased Marshall’s share of her fortune.

Defense lawyers say that although she was suffering from Alzheimer’s, Astor had lucid moments.