The Ethos Effect (2003) is a science fiction novel by American writer L. E. Modesitt, Jr., a sequel to The Parafaith War.

Many factions including the Eco-Tech Coalition, the Revenants of the Prophet ("revs") and the Taran Empire are engaged in escalating conflict over territory and their competing social philosophies.

This is one of the only books written by Modesitt where a person enacts major political and structural changes in the culture into which the protagonist was raised.

Jackie Cassada in her review for The Library Journal called this novel a "fast-paced yet complexly plotted work of military sf portrays three-dimensional characters caught up in internal and external battles as they search for ways to combine personal integrity with their professional duties.

"[1] Peter Cannon in his review for Publishers Weekly said that "despite some expository lumps and wooden characterization, thoughtful SF readers will appreciate this weighty tale of humanitarian intentions and social speculations.