In J. R. R. Tolkien's fictional realm of Middle-earth, the Olog-hai were a fierce breed of Trolls that appeared during the last days of the War of the Ring in Sauron's service. They had none of the old Troll vulnerabilities: they were very intelligent and able to move under the sun. For this reason they were seen by some to be Orcs like the Uruk-hai, but they were definitely of Troll stock.

They only spoke Sauron's Black Speech, and it is possible they only existed because Sauron's will empowered them.

The Olog-hai do not actually appear in The Lord of the Rings and are only mentioned in the appendices to the book. It is however possible that some Olog-hai were present at the Battle of the Pelennor Fields. They were shown as the shock troops in The Lord of the Rings: The Return of the King (movie) motion picture, sporting armor and the famed war-hammers alluded in the appendices. These Olog-hai, were not shown talking in any speech short of grunts. They differ from the other trolls depicted in the movies in that they have more forward-facing eyes as well as hair on their bodies, alluding to the fact that they are a more advanced form of troll. Their eyes are also bright orange, showing their evil connection with Sauron.