Borough councillor for Rowner and Holbrook Patrick Bergin said his motivation had shifted following the EU referendum to "managing the exit".
Mr Bergin said he formally announced his decision "before anyone was aware" that UKIP leader Diane James had stepped down.
He said he would continue to represent his ward as a Conservative councillor.
He said: "I know that to help the people in my ward and be part of decision making and take Gosport forward, I can only do so by being an active member of a group who have the vision to take Gosport forward.
"That group is the Conservatives."

Patients at Royal Stoke University Hospital were taken outside in their beds when a fire broke out in the main building on Wednesday evening.
The incident followed a fire in toilets at Staffordshire University earlier in the day, police said.
Thomas Ashcroft, 39, of Musgrave Crescent, Sheffield, has been charged with two counts of committing arson with intent to endanger life.

Jermaine Easter nodded Rovers ahead from Lee Brown's cross before Ollie Clarke doubled the lead with a low right-footed shot.
Jon Meades' header gave Wimbledon hope but Matt Taylor's 12-yard finish, his 19th of the season, sealed victory.
Wimbledon, who are seventh, had substitute Adebayo Akinfenwa sent off for dissent.
Fifth-placed Rovers are six points off an automatic promotion spot with 11 games to go.
AFC Wimbledon manager Neal Ardley told BBC Radio London:
Media playback is not supported on this device
"When you assess it afterwards it's not just the points you haven't gained, it's injuries and now suspensions."
"As far as the actual game goes, I thought we were really good. They were ruthless with their chances, we made some silly defensive errors at certain times and they punished us."
"I can't ask for any more. I thought in the second half particularly we ran all over them."

Melvyn Marcel was appealing against the decision, made by a judge earlier this year, that burials should be organised for his mum Hilda and his dad Eugenios.
However, appeal judges at the Court of Session rejected his argument.
Eugenios and Hilda Marcel's bodies were found in a former fishmonger shop in 2002.
In February, councillors gained permission from Lord Mullholand to arrange a burial for the couple.
Mr Marcel wanted to build a fridge at his Edinburgh home then plans to build a private mausoleum in the grounds of his property.
He eventually planed to take his parents' bodies to be buried in the West Bank in the Middle East.
The council argued they had a statutory duty to dispose of the bodies.
On Friday, Mr Marcel told the court Lord Mulholland's decision was wrong.
He told judges Lord Carloway, Lord Brodie and Lady Clark of Calton: "I'm fighting for justice but nobody's helping me."
Mrs Marcel died in 1987 from lung cancer whilst her husband passed away from prostate cancer aged 91 in 1994.
Their bodies were embalmed and a relative regularly visited them at the shop in Polwarth.
Police discovered Mr and Mrs Marcel's bodies during an investigation into alleged fraud at a funeral home in West Lothian.
It was claimed that staff at the Broxburn undertakers had been paid to preserve the remains and four employees were sacked.
However, none of those who were allegedly involved in the fraud were charged or prosecuted for any offence.
Since then the Marcels' bodies have been stored at Edinburgh City Council's mortuary with officials unable to find them a more suitable resting place without the consent of the family.
Giving the appeal court's decision, Lord Carloway said that Lord Mulholland's decision was legally correct.
He added: "No valid defence has been established in this action.
"It is the court's decision to adhere to the Lord Ordinary's interlocutor."
A UKIP councillor in Gosport has announced he is defecting to the Conservative Party.
[NEXT_CONCEPT]
A man has been charged with arson after fires at a hospital and university.
[NEXT_CONCEPT]
Bristol Rovers strengthened their promotion hopes with victory over AFC Wimbledon, their third in a row.
[NEXT_CONCEPT]
The son of a couple whose bodies have spent almost 15 years in an Edinburgh mortuary has lost a legal bid to stop the council arranging their funerals.