The Hawks appointed the former Dartford and Gloucester City manager in September following a slow start to the season.
But the National League South side are winless since August and last week passed the three month mark since their last victory.
Havant are 14 points from safety and Cliff De Gordon, Ross Betteridge and Richard Pope will be in charge when they face fellow strugglers Dover Athletic on Saturday.
A club statement read: “Following on from a meeting of the Board of Directors, the club can confirm that First Team Manager, Steve King, has been relieved of his duties with immediate effect.
“As a result, tomorrow’s Vanarama National League South fixture at Westleigh Park against Dover Athletic will be overseen by our Under 23’s Management Team consisting of Cliff De Gordon, Ross Betteridge and Richard Pope alongside current First Team Goalkeeping Coach Alan Walker-Harris.
“The Board recognises that there needs to be an upturn of results and will conduct a full and thorough recruitment process for the next Havant and Waterlooville Management Team. The club’s board wishes to thank Steve King for his tireless efforts in attempting to turn around the fortunes of the first team and wish him success in any future role.
“The board would like to place on record its thanks to our loyal supporters for their continued and unwavering support during a long run of difficult results for the football club. It is right now that we need the support of each and every one of you starting at Westleigh Park tomorrow.”