The new mid-morning departure from Abu Dhabi and late-evening departure from Heathrow will get under way on 27 October.
It follows the introduction of additional seasonal services by Etihad this summer.
Etihad will deploy a two-class Boeing 787-9 Dreamliner on the route offering 28 business studios and 271 economy seats.
Robin Kamark, Etihad commercial officer, said: “Adding a fourth year-round flight will provide much needed capacity and optimised timings and easy connections to key destinations across the Middle East, Africa, Asia and Australia.
“The new service demonstrates our commitment to the crucially important UK market.”
