Adam Fenix (voiced by Peter Renaday) is Marcus Fenix's father. Fenix is hailed as one of Sera's greatest minds, whose radical and innovative research greatly influenced the COG's technology.

In the novel Gears of War: Aspho Fields, set several decades prior to Gears of War, Adam joins the COG army during the Pendulum Wars, rising to the rank of major while gaining a considerable reputation. When Marcus joins the COG army himself, Adam tried to use his influence to enroll Marcus in an officer's academy, but Marcus refuses; Adam's disappointment in Marcus' career was a point of tension in their relationship. Ten years after E-Day, Adam is supposedly killed during a Locust offensive on the COG capital, Ephyra. In a flashback in Gears of War 3, Adam is buried under falling debris at the Fenix Estate as Marcus and Dom try to rescue him.

During the fourth act of Gears of War 2, Delta squad discovers recordings of Adam detailing a plan to kill the Locusts by deliberately sinking Jacinto to flood Nexus with water. At the conclusion of the game, after the COG have sunk Jacinto, a transmission from Adam Fenix is played at the end of the credits asking, "This is Adam Fenix, is anyone out there...? Can you hear me...? This is Adam Fenix, can you hear me...? What have you done...?" Adam Fenix is a main part of Gears of War 3 as the game focuses on Marcus trying to find his father. Marcus learns that his father is alive, but is held captive on the top-secret research island of Azura to find a way to kill the Lambent under Prescott's orders.

When Azura is taken over by the Locust, Adam's life is spared in order for him to refine his Imulsion-destroying weapon so it will not affect the Locust and will eradicate out the humans as well. Adam manages to send a message to Marcus through Prescott and Delta Squad comes to Azura with an army to retake the island and implement his solution. Adam guides the squad through Azura using security cameras to monitor their progress and informs them of what he knows. Adam is rescued by Delta and they defend him from Myrrah as he activates his weapon. Adam succeeds in activating his weapon and wipes out the Locust, Lambent and Imulsion, but dies as well as he infected himself with Imulsion to test its life-cycle. Adam's last request is that his son live his life with no regrets or looking back.

Epic Games announced on May 16, 2011 that players who purchase the Limited or Epic Edition will receive a code on Adam Fenix's medal to unlock Adam Fenix as a playable character in multiplayer.