John Steinbeck's last completed novel was "The Winter of Our Discontent" (1961). However, he died before completing his final novel, "The Acts of King Arthur and His Noble Knights" (published posthumously in 1976).
It's important to note that "The Winter of Our Discontent" was Steinbeck's last published novel in his lifetime. He was working on "The Acts of King Arthur and His Noble Knights" at the time of his death, but he only completed about half of it.