The 22-year-old was told by boss Michael O'Neill on Wednesday that he had made the cut for France, ahead of today's squad announcement.
McCullough is likely to provide defensive cover during his country's first appearance at a major tournament since 1986, despite finishing the domestic season in fine form for Doncaster playing in midfield.
Northern Ireland start their campaign against Poland in Nice on June 12.
They face Ukraine in their second Group C game in Lyon on June 16 and finish against world champions Germany in Paris five days later.
Northern Ireland Euro 2016 squad
Goalkeepers: Roy Carroll, Michael McGovern, Alan Mannus.
Defenders: Jonny Evans, Gareth McAuley, Craig Cathcart, Conor McLaughlin, Shane Ferguson, Lee Hodson, Luke McCullough, Aaron Hughes.
Midfielders: Stuart Dallas, Chris Baird, Jamie Ward, Steven Davis, Oliver Norwood, Corry Evans, Niall McGinn, Paddy McNair.
Forwards: Kyle Lafferty, Conor Washington, Josh Magennis, Will Grigg.