The trustees of Wykeham House School in Fareham had warned last month its future was in doubt.
A statement blamed falling numbers and said most children had found alternative schools.
The school offered single sex teaching for both boys and girls, charging fees of up to Â£4,100 per term. Both the boys' and girls' sites are to close.
Wykeham House School was established, initially as a girls' school, in 1913 and has about 120 pupils.
A new boys' senior school site opened in September 2014.
Its website said the school prided itself on its "small classes and nurturing environment".
Chair of trustees John Charles, said: 'We are sorry to be closing the doors on such a wonderful school.
"We have worked hard to explore different options to secure a future for our school but in the face of reducing pupil numbers and a shrinking market place for single sex education we have had to take the sad decision to close the school.'
The school statement said it would close at the end of the autumn term.

Mandla Hlatshwayo and his friend were shot after confronting a group of men who had robbed women of their mobile phones in a pub in Soweto.
The suspects were found in possession of drugs and an unlicensed gun.
South Africa has one of the highest murder rates in the world with more than 50,000 cases reported every year.
"Police are questioning the suspects to see if they can link them to the shooting," says the police's Lungelo Dlamini.
Tributes are still pouring in for the star who was also a DJ on local radio station Jozi FM.
Those who knew the 40-year-old have described him as a selfless man.

Favourite Air Force Blue, winner of the Dewhurst Stakes last year and four of his five races, never challenged and was a distant 12th of the 13 runners.
Dettori crossed to the rail side to win by one and half lengths for his third victory in the prestigious mile race.
Massat (9-1) was second with Ribchester (33-1) two lengths back in third.
Air Force Blue was widely expected to give trainer Aidan O'Brien a record eighth 2000 Guineas success.
But Ryan Moore could find no response from the bay colt and it was Dettori who surged clear, 20 years on from his first success in the race on Mark of Esteem.
He also won again in 1999 on Island Sands.
The veteran jockey, 45, said of his latest triumph: "We had a bad draw but he's a great horse.
"His main forte is that he stays, he galloped out really strong."
Trainer Hugo Palmer said: "Frankie was so alert, he said nothing had been coming from behind and that if nothing took us on, he would do it himself and he did.
"There were no hard luck stories, for us anyway. It was a masterful ride."
BBC horse racing correspondent Cornelius Lysaght
"Considering the going was on the soft side, the time was decent so that's another plus about a success achieved by a decisive length and a half.
"The winner had to race on the more challenging outside of the field because he was drawn in stall one, and Frankie Dettori gave him extra credit for the performance because of that.
"This was Hugo Palmer's first British Classic win; he's very much an emerging force - it won't be his last."
An independent school in Hampshire has confirmed it will close because of "financial pressure".
[NEXT_CONCEPT]
South African police say four people have been arrested in connection with the murder of former actor on popular local TV series Generations.
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Frankie Dettori rode 14-1 shot Galileo Gold to a surprise victory in the first classic of the 2016 Flat season, the 2000 Guineas at Newmarket.