The 29-year-old Porter, who was in the final year of his contract, had been the starting right outside linebacker since 2000, the year after he was drafted by the Steelers.
Porter was to receive a $1 million bonus on March 6, and was due to earn $4 million in 2007.
"It's a business decision that was made," Steelers spokesman Dave Lockett said.
Director of Football Operations Kevin Colbert said the salary cap was the reason Porter was cut.
Releasing Porter allows the Steelers to get under the NFL's $109 million salary cap by Friday, when the free agent signing period begins.
"Releasing a player like Joey Porter, who has meant so much to this franchise and helped us win a Super Bowl championship, is not an easy thing to do," Colbert said. "It's definitely a salary cap-related issue where we needed some short-term relief. But we also had to factor in what our cap situation's going to be in 2008 and 2009 and beyond. ... I'm sure Joey will attract a lot of interest (from other teams) very quickly."
Porter, who was signed through 2007, says there are no hard feelings.
"The organization has always been good to me," Porter said in a television interview. "It was a great run in Pittsburgh. I have no problem with them at all. I think it was a tough decision that they had to make, and it was one they had to make and they did it in the right way."