ESPN's Shelley Smith also reported the pending move through independent sources.
Whether LeBron James, the kingpin of this summer's celebrated free-agent class, will join them remains to be seen. James will announce his decision at 9 p.m. ET Thursday during a one-hour special on ESPN.
Wade and Bosh are expected to announce their decision on Wednesday afternoon, according to the source, and continue to lobby James, along with Heat president Pat Riley, to join them in Miami, despite the financial complexities involved for the Heat to make room for all three.
Both players are expected to get the maximum amount allowable under the league's collective bargaining agreement, though the addition of James could change the players' salaries. Contracts cannot be officially signed until Thursday.
It was not immediately clear whether Bosh -- who spent seven seasons with the Toronto Raptors -- will sign with Miami outright or join the Heat through a sign-and-trade deal. Bosh could earn $125 million over six years via sign-and-trade, but only $96 million in five without it.
"Either way, he's definitely going to Miami," the source said.
League sources say Riley is trying to convince Bosh to take the five-year max deal and leave Toronto outright. The Raptors are trying to persuade Bosh to demand a six-year deal and a sign-and-trade so that they get something out of the deal.
Earlier this week, the Raptors reportedly had been reluctant to do a sign-and-trade, especially with Miami, contending the Heat did not have any players of interest. But if Bosh leaves outright, the Raptors are left with, at most, $4 million in cap space to replace Bosh.
One source with knowledge of Toronto's thinking told ESPN.com's Marc Stein that the Raptors, as of 9 a.m. ET, had yet to hear anything official from Bosh's camp about his intentions.
Raptors general manager Bryan Colangelo, in a text message to The Associated Press on Wednesday morning, said, "I have nothing official from anyone."
Amid a growing sense among rival executives that James intends to stay with his hometown Cleveland Cavaliers, sources close to the situation told ESPN.com that James is still seriously considering joining forces with Wade and Bosh in Miami and had a discussion about the possibility that all three could wind up playing together for the Heat on a conference call early Wednesday morning.
James or not, the Heat could emerge as one of the top teams in the Eastern Conference with Wade and Bosh aboard.
Wade, Bosh and James have maintained a dialogue throughout the free-agent process and in the days leading up to July 1, keeping alive the possibility that all three could wind up playing together for the Heat while also sifting through the various options on the table.
ESPN.com reported June 28 that the trio held a scaled-down version of the free-agent summit originally suggested by Wade to seriously discuss their futures, with a focus on the idea that all three could team up with the Heat. Sources close to the situation insist, though, that James has not promised anything to his Team USA colleagues and is not tipping off which way he's leaning.
For much of Monday and Tuesday, several team executives and league insiders expressed their belief to ESPN.com that James re-signing with the Cavaliers, and Wade and Bosh landing together in Miami, was the most likely outcome of a saga that began three years ago when James, Wade and Bosh all signed contracts that enabled them to become free agents in the summer of 2010.
But the New York Knicks, after securing a verbal commitment from free agent power forward Amare Stoudemire, have not surrendered in the chase for James, despite a growing sense around the league that the opportunity to play with Wade and Bosh is the only scenario that prevent the Cavs from keeping their face of the franchise.
Chris Broussard is an NBA writer for ESPN The Magazine. ESPN The Magazine's Ric Bucher, ESPN.com senior NBA writer Marc Stein and The Associated Press contributed to this report.