May 26, 2016
The Associated Press reported Thursday morning that Donald Trump has reached the 1,237 delegates needed to clinch the Republican nomination.
While this was not expected until June 7th, "Trump was put over the top in the Associated Press delegate count by a small number of the party's unbound delegates who told the AP they would support him at the national convention in July."
These unbound delegates are akin to superdelegates on the Democratic side, although far fewer in number. The AP query netted additional unbound delegates from North Dakota and Pennsylvania, among others.
Trump will be officially nominated at the Republican National Convention in Cleveland the week of July 18th.