Ian Douglas (born 1950) is the pen name of American author William H. Keith Jr., a prolific writer known for military science fiction and space opera. He has also written under several other pen names and produced dozens of novels across science fiction, fantasy, and media tie-in fiction.

Ian Douglas is regarded as a dependable and influential writer within military science fiction. While not heavily award-focused, his long-running series and detailed future history timelines have helped shape the modern space military fiction subgenre.