Denis McCullough (1883 - 1968) was a prominent Irish rebel in the early 20th century.

Born in Belfast, McCullough was a separatist from an early age. When he was 17, his father had him inducted into the Irish Republican Brotherhood at the side door of a pub by a man who seemed to view the ritual as an unpleasant distraction to a night of drinking. The event disillusioned McCullough with the Brotherhood, and he soon took it upon himself to revitalize the organization.

He did so over the years with the aid of Bulmer Hobson and Sean MacDermott. Together they founded the Dungannon Clubs for recruitment into the Brotherhood, and they worked to remove the "armchair republicans" from positions of power to be replaced with more determined men. Their cause prospered with the return of veteran Fenian Tom Clarke to Ireland in 1907.

McCullough was elected to fill the vacant seat of the President of the IRB late in 1915, a position he held during the Easter Rising of 1916, though he took no active role in the rising itself. He was not a member of the Military Committee that was responsible for its planning (and probably didn't even know of its existence until after the rising). It is likely that the other members of the 3-person IRB executive, Clarke and MacDermott (the treasurer and secretary) supported his nomination as president because, being isolated in Belfast, he would be in no position to interfere with their plans. Nevertheless, during Holy Week he got word of what was afoot and travelled to Dublin to question Clarke and MacDermott, who avoided him as long as they could. Eventually they informed him of their plans, which he was brought to support.

Though he was a member of the Irish Volunteers, it was decided that Belfast could not take part in the rising, as the dominance of the Ulster Volunteers in the northeast would lead to civil war. Therefore McCullough was to lead Volunteers in his area to Connacht. When the Volunteer's Chief-of-Staff Eoin MacNeill issued a countermand, cancelling orders for the rising, McCullough remained in Belfast. Nevertheless he was arrested that week and spent several months incarcerated. He retired from politics in 1927.

It has been argued that as President of the Irish Republican Brotherhood at the time of the Easter Rising, the title President of the Irish Republic was by rights his, and not Patrick Pearse's. However, as he had no real role in the planning of the insurrection, and was not in the vicinity of Dublin, where it was clear the leadership would need to be, it is understandable that Pearse was given the title instead. McCullough was likely glad to not have the title, as it certainly would have meant his execution along with the other leaders.