US daily The Wall Street Journal cited sources familiar with her plans and said she would be making an appearance to deliver remarks at around 10 a.m. ET (1500 GMT).
The expected decision clears the way for Donald Trump as the Republican presidential candidate who will most likely once again face-off with Democratic President Joe Biden in November.
Haley, who served as the US ambassador to the UN under Trump, became his most significant rival in the Republican presidential race.
Trump won 14 of the 15 primary elections in the "Super Tuesday" states, including Texas and California, but did not manage a clean sweep with Haley's surprise upset in Vermont.
According to the WSJ, Haley is not expected to announce an endorsement on Wednesday, but will rather encourage Trump to earn the backing of Republican and independent voters who supported her.
