"Portsmouth Football Club can confirm that Tony Adams and John Metgod have left their positions as manager and first team coach with immediate effect," the statement said.
Portsmouth's 3-2 home defeat to Liverpool on Saturday spelled the end of Adams' short reign at the club, who are hovering one point above the relegation zone.
Adams, a former England captain, won just four of his 22 matches in charge after being promoted from assistant manager when Harry Redknapp left to join Tottenham in October.
Portsmouth, the holders, were also knocked out of the FA Cup and eliminated from the UEFA Cup.
Paul Hart, the club's director of youth operations, will take over as caretaker manager until a new boss is appointed.
Reports said Alan Curbishley and ex-Chelsea manager Avram Grant were on a shortlist to take over, although Curbishley might be prevented from doing so as he is embroiled in a legal wrangle with his former club West Ham United.
A Portsmouth spokesman said on the website: "This has been a very difficult decision and Tony has worked tirelessly to arrest the slump in form.
"He is rightfully highly respected within the football world and played a major role in our FA Cup triumph last season.
"We would like to thank him and John and wish them well in the future.
"The owner and the board feel they have been as supportive as they can during a period where results have been poor.
"The team has played well but too many points have been dropped from winning positions," the statement said.
Portsmouth were twice ahead against Liverpool on Saturday before falling to defeat.
The spokesman added: "The priority is for Portsmouth Football Club 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."