John G. Hemry (born 1956) is an American science fiction author best known for writing military science fiction under the pen name Jack Campbell. A retired U.S. Navy officer, his military experience strongly influences the realism, leadership themes, and tactical detail found in his novels.

Hemry’s main impact has been in helping modernize military science fiction by emphasizing realistic command decisions, the psychological burden of leadership, and the physics of space warfare rather than purely action-driven combat. His work is often compared to earlier military sci-fi traditions established by writers like Robert Heinlein, but with a stronger focus on operational realism and ethics.

John G. Hemry is regarded as one of the more important contemporary writers in military science fiction. Through his Lost Fleet universe in particular, he has helped shape modern space naval fiction and built a strong readership among fans who appreciate realistic depictions of leadership and military strategy in science fiction.