The university didn't say why Armstrong was kicked off the team but did say he could stay enrolled at the University of Miami as a scholarship student to complete his degree.
UM said it would grant the senior a full release if he decides to transfer to another school.
Armstrong's dismissal Wednesday morning came about a month after InsideTheU.com, which covers Miami sports teams, reported he had discussed on Twitter an interaction with a booster.
Armstrong's father, Albert Armstrong, said he did not know his son's plans.
If UM had revealed plans to dismiss him a few weeks ago, Armstrong could have entered the NFL's Supplemental Draft last Thursday, the Herald said.
The newspaper said he's now more likely to transfer to another school, where he would play his senior year.
Armstrong had been suspended by the NCAA for the first four games of the season for accepting impermissible benefits from former booster Nevin Shapiro. The NCAA is investigating Shapiro's relationship with UM.
Armstrong was suspended for one game in November after going to dinner at a Miami Beach steakhouse with a friend who owns a public relations company that works with professional athletes, then posting on Twitter about the dinner. The Herald said Armstrong was later cleared of wrongdoing related to the dinner or Twitter post.