Associated Press’s delegate count has him reaching the quota thanks to a small number of the party’s unbound delegates.
Mr Trump, who defeated 16 other Republican contenders, reportedly has 1,238 delegates – one more than needed.
Republicans are expected to finalise their nomination at a convention in Cleveland in July and the New York real estate mogul will not be officially named as the Republican hopeful until the event.
While his closest presidential rivals – Ted Cruz, John Kasich and Marco Rubio – have yet to free delegates pledged to support them, Mr Trump can win the prize with or without their help.
Mr Trump’s Republican rivals have long called him a “New York liberal” who can’t be trusted to uphold their Republican values. Mr Trump describes himself as a “commonsense conservative” and his message has earned him millions of Republican votes.