John Jacob Astor 5th, a descendant of one of America's most fabled merchant princes, died yesterday at his home in Miami Beach. He was 79 years old.

His death was reported by two of his cousins, R. Thornton Wilson, and Stephen Spencer, who said they did not know the cause.

Mr. Astor was born in New York in August 1912. His father was John Jacob Astor 4th, a businessman and inventor, who built the Astoria Hotel in New York City that was later combined with the hotel next door to become the Waldorf-Astoria. His mother was the former Madeline Talmage Force of New York City. She was pregnant with him when she and her husband sailed on the Titanic. Her husband put her on a lifeboat and went down with the ship on April 15, 1912.

Mr. Astor 5th, one of whose half-brothers was Vincent Astor, the New York philanthropist, attended St. George's School, in Newport, R.I. Mr. Spencer said that his cousin never worked after he was dismissed from a job he held briefly after graduating from St. George's in the early 1930's.

Mr. Astor was married three times and divorced twice. His first wife was Ellen Tuck. His second wife was Gertrude Gretch. His third wife, Sue Sandford, died several years ago, Mr. Spencer said.

Mr. Astor is survived by a son, William; a daughter, Jacqueline Drexler of Manhattan; a brother, John H. Dick of Meggett, S.C., and three grandchildren.