"I am humbled and honored to be joining this incredible project and to be able to tell the story of the well respected young mutant Nightcrawler," the 21-year-old actor wrote on his own Instagram, where he thanked Singer and "the passionate X-Men fans."
Smit-McPhee joins recent cast additions Alexandra Shipp as Storm, Sophie Turner as Jean Grey and Tye Sheridan as Cyclops in the 1980s-set Fox sequel, stepping into the roles originated by Halle Berry, Famke Janssen and James Marsden. "X-Men" veterans James McAvoy, Michael Fassbender and Jennifer Lawrence are also set to return, with Oscar Isaac playing Apocalypse.
Singer has teased the possibility of Nightcrawler's return to the franchise for nearly a year, revealing he'd actually written a scene featuring the character for "Days of Future Past" before ultimately scrapping it. Alan Cumming originated the role in 2003's "X2."
Smit-McPhee, whose credits include "The Road," "Dawn of the Planet of the Apes" and "Young Ones," stars opposite Fassbender in the Western action thriller "Slow West," which premiered last month at Sundance.
"X-Men: Apocalypse" arrives May 27, 2016.