The 19-year-old, who started out his career with Portadown before moving to England in 2020, has been without a club since leaving Turf Moor this summer, where he played for their U18 and U21 sides.
McCullough also enjoyed a short-term loan spell with Hyde United earlier this year and has represented Ntorhern Ireland U19s on four occasions.
“It feels great (to sign for Linfield),” he told the club’s media channel. “It has been a long time coming.
“I’ve been without a club for three or four months now, so to get this over the line is really good.
“I like to get forward but also love to defend too.
“I’ve been away playing academy football for a few years but now it’s time to really get into men’s football and time to let people know who I am.”
McCullough had been training with the Blues, who currently top the Sports Direct Premiership standings, during the summer and feels playing under manager David Healy can help bring the best out of his game.
