The 21-year-old, who will wear the number 10 shirt, arrives with valuable experience after previous loan spells at Leyton Orient, Stoke City and Oxford United.
Donley joined Tottenham’s academy at eight and progressed through the ranks, representing England from Under-15 to Under-19 level before switching allegiance to Northern Ireland.
“Jamie's a quality player and someone who we have kept our eye on since January. He went to the Championship with Oxford, but we’ve maintained a relationship with him,” said manager Jack Wilshere
“One thing we've done really well in this window is that we’ve had targets we wanted to go after.
“We knew the positions we wanted to improve or add depth in, and Jamie’s definitely been top of that list for a long time, so to get it done feels like a really important signing.”