ESPN's Adrian Wojnarowski reported the deal sending the 30-year-old to the Blazers on Monday. This comes a day after the Heat agreed to acquire Jimmy Butler in a sign-and-trade from the Philadelphia 76ers.
Following a breakout 2015-16 season, Whiteside was rewarded with a four-year max contract worth $98.4 million by the Heat.
Originally drafted by the Sacramento Kings in 2010, he bounced around what is now the G League and international teams before joining the Heat prior to the 2014-15 season. He has averaged a double-double with 14.1 points and 11.9 rebounds per game in five seasons with Miami.
Heat president Pat Riley said in February 2017 he felt Whiteside was an integral part of the roster, though he wanted to see his center work on a few things to become an even better player.
"I love Hassan," Riley told Shandel Richardson of the Sun Sentinel. "We would not have invested in Hassan Whiteside if we didn't believe that he could be an integral part of winning, a force on defense and rebounding, from an offensive standpoint. There still are parts of his game that he has to work on and how he incorporates his game with teammates."
Following an overtime loss in March 2018 against the Brooklyn Nets, Whiteside vented his frustration with head coach Erik Spoelstra after playing just 20 minutes and sitting out the entire fourth quarter and overtime.
"A lot of teams don't have a good center," he told reporters. "They are going to use their strengths. It's bulls--t. It's really bulls--t, man. There are a lot of teams that can use a center."
The North Carolina native also left the bench early after only playing 25 minutes in a Dec. 4 loss to the Orlando Magic, though he told reporters it was to use the restroom. He was relatively quiet after that moment, averaging his usual double-double with 12.3 points and 11.3 rebounds in 72 games.
With the Heat struggling to find answers after winning the Southeast Division title in 2017-18, moving Whiteside's contract for Leonard and Harkless was their best way to find a path forward.
The Blazers will hope Whiteside can return to form as a defensive stopper in the middle as they look to build on their conference finals run last season.