Niwa wrote novels, short stories and essays, depicting dark family tangles and vivid narratives of modern life. In his later years, he tackled the life of Shinran, a Buddhist monk who lived from 1173-1262, and other weighty subjects.
Niwa was awarded the Order of Culture in 1977.
Niwa was born in 1904. His father was a Buddhist monk at a temple in Yokkaichi, Mie Prefecture. His mother left when Niwa was 8, and his yearning for her shaped his later writings.
After graduating from Waseda University in Tokyo, he briefly lived the life of a monk in Mie Prefecture.
His acclaimed debut novel, ``Ayu'' (Sweetfish), about his mother, was published when he was 28. A string of best sellers followed. Most depicted well-to-do married women and the glitter of city life, featuring insights into human weakness.
During the war, his novels were banned as too racy.
He continued writing after the war. In 1947, ``Iyagarase no Nenrei'' (The hateful age), the tale of a spiteful old woman, topped the sales charts. The book's title became a catchphrase.
Niwa began to re-examine religion and human destiny in the latter part of his career.
His five-volume biography of Shinran took five years to write, followed by an eight-volume saga about the 15th-century monk Rennyo.
He was diagnosed with Alzheimer's disease in 1986.