Porter was in his second season with the Astros and was succeeded by interim manager Tom Lawless, who worked in Houston's minor league system. Lawless' first game in charge is Tuesday night at home against the first-place Los Angeles Angels.
Bench coach Dave Trembley also was let go.
Porter joined the Astros after working as a third-base coach for the Washington Nationals. The Astros went a franchise-worst 51-111 in his first season for their third straight 100-loss season.
General manager Jeff Luhnow said the decision was not based on "our current level of competitiveness." The Astros entered Monday in fourth place in the AL West with the second-worst record in the league.
"I recognize that our win-loss record is largely a product of an organizational strategy for which I am responsible," Luhnow said in a statement. "Rather, I made this decision because I believe we need a new direction in our clubhouse."
Luhnow refused repeatedly to get into more detail about why Porter was let go as he fielded questions for almost 25 minutes on Monday afternoon. He did say that he had been mulling the move for a few weeks and that he made the final decision on Sunday.
"Once you make a decision it is in everyone's best interest to move it along," he said. "It's easier to wait until the end of the season in some respects, but it's not fair to Bo and it's not fair to us."
Owner Jim Crane said the firing was "not an easy decision to make." It comes following recent reports citing sources who said Luhnow and Porter were not getting along. Luhnow wouldn't address those rumors other than to say that there were "numerous inaccuracies" in the reports.
Porter issued a statement Monday afternoon.
"During my time in Houston I dedicated myself to do everything I could to help this organization win, in the short-term and for the long-term," he said in the statement. "I am proud of what we were able to accomplish in Houston with an organization in transition. I'm gratified we were able to bring some excitement to this city as a result of our improvement from 2013 to 2014."
Starting pitcher Scott Feldman lamented Porter's firing and noted the care and concern he showed when Feldman's father died earlier this season. He said he spoke to Porter on Monday and thanked him for the way he treated him throughout the season and particularly in that difficult time.
"I hope he realizes that he was doing as well as he could given the circumstances," Feldman said. "It's tough to see, but Bo's a strong guy and I'm sure he'll be back on his feet and back in the game whenever he wants to be."
Whoever is hired as full-time manager will be the team's fourth since 2007, not including those who held the job on an interim basis. Luhnow said they hope to hire someone quickly.
"What we will seek going forward is a consistent and united message throughout the entire organization," Luhnow said. "It is essential that as an organization we create an atmosphere at the major league level where our young players can come up and continue to develop and succeed.
"Ultimately, I am responsible for creating that culture, and I will do everything in my power to do so -- even when it means making difficult moves like the one we made today."
Porter was the first managerial hiring in Luhnow's tenure, and he was unhappy it didn't work out.
"It is a disappointment in that any time you bring in a new leader of any department ... you hope that the individual is going to be there for the long haul," he said. "In no way did we intend to hire someone for a short-term (transitional) period. As we look forward to the next hire, we're looking for someone for the long haul."
He said many people will help in the hunt for a new manager and noted that Hall of Fame pitcher Nolan Ryan, who re-joined the team in February as a special assistant following a stint with the Rangers, will be among them.
The firing is the latest development in months of trouble surrounding the Astros that includes being the victim of hackers who accessed team's servers and published months of internal trade talks on the Internet, and failing to reach a deal with No. 1 overall draft pick Brady Aiken.
"It's been a challenging summer for me personally and for us as the Astros organization," Luhnow said. "A lot of things happened not the way we scripted them to happen and we've had to react. That's part of the job."
Lawless was the interim manager at Triple-A Oklahoma City this season when manager Tony DeFrancesco was on medical leave. Adam Everett, who was also working in the minors, will replace Trembley as bench coach.