Was fat man part of the Manhattan Project?

Yes, "Fat Man" was part of the Manhattan Project. It was the code name for the atomic bomb dropped on Nagasaki, Japan, on August 9, 1945. The bomb used plutonium as its fissile material and was developed alongside "Little Boy," the bomb dropped on Hiroshima. Both bombs were a result of the extensive research and development efforts of the Manhattan Project during World War II.