The 23-year-old, multiple golf-major winner is thought to have been in discussions with Nike over the proposed 10-year-deal.
It is expected that an agreement could be signed within the next few months.
One senior source from a sports sponsorship agency said: "No doubt this is going to happen. And it could be worth up to $25m (£16m) a year."
That would be similar in size to Tiger Wood's deal with Nike.
McIlroy currently has endorsements deals with golf brand Titleist, Oakley, the hotel chain Jumeirah, luxury watchmaker Audemars Piguet, and Santander.
McIlroy’s deal with Titleist expires at the end of the year.
Nike's strategy is for the golfing stars it endorses to be "clean", so that the Nike swoosh is the only visible brand on their clothing. It is likely that Nike would buy out any existing endorsement deals with McIlroy that have time running on them.
In 1996, Woods signed a deal valued at $40m (£25m) over five years, which was then extended to $105m over the same period.
Nike recently dropped its sponsorship of disgraced cyclist Lance Armstrong
Nike declined to comment. McIlroy’s agent Horizon Sports was unavailable for comment.Follow @johnreynolds10