"The owner and the board feel they have been as supportive as they can during a period when results have been poor," the club said in a statement."The priority for the club is to remain in the top flight and we feel the appointment of a new manager will give us the best opportunity to enable us to do this."
Adams replaced Harry Redknapp at the helm last October following his move to Tottenham Hotspur but under his stewardship, FA Cup holders Pompey have fallen to 16th place, just one point above the relegation zone.
The final straw came on Saturday when Portsmouth conceded two goals in the last five minutes to lose 3-2 at home to Liverpool.
"The team has played well but too many points have been dropped from winning positions," the statement read.
Director of youth operations Paul Hart will take over as caretaker manager following Adams's sacking and that of first-team coach John Metgod.
Adams had been at Portsmouth since 2006 when he began a role as Redknapp's assistant. Prior to that, the 42-year-old had a brief spell as manager at Wycombe Wanderers, as well as coaching jobs at Dutch clubs Feyenoord and Utrecht.
The Englishman told BBC radio said he was disappointed to lose his job after such a short reign.
"I am a little bit surprised but results haven't been good," he said.
"I haven't had a lot of time and I certainly haven't had the financial backing. We've had a few injuries as well and that's tough to take.
"I'm still up for the challenge so I've got mixed feelings."
"We have been making a lot of mistakes. We are in the results industry and the owner and chairman will point to that. I'm not going to bang on about excuses.
"I think I could have turned it around but it was not to be."
Former West Ham United boss Alan Curbishley and ex-Chelsea manager Avram Grant are believed to be in the frame as possible long-term replacements.