Assad Uzzaman, 25, was one of five men who left the UK in October 2013 to fight as jihadists.
His death has been reported by academic Shiraz Maher who tracks the movements of foreign fighters in Syria and Iraq.
Three of the Portsmouth group have also been killed. A fifth was jailed when he returned to the UK.
Mehdi Hassan, Mamunur Roshid and Hamidur Rahman have previously died in Syria.
Mashudur Choudhury returned to the UK after a few weeks and last year became the first person in the UK to be convicted of terrorist offences in connection with the conflict in Syria.
Another Portsmouth man, Ifthekar Jaman, left in the spring of 2013 and was also killed in fighting.
Uzzaman's death was reported by Shiraz Maher, a senior research fellow at the International Centre for the Study of Radicalisation (ICSR) at King's College in London.
He said: "The last of the Portsmouth cluster of fighters with Islamic State, Assad Uzzaman (Abu Abdullah) has died."
In a statement, Flick Drummond, Portsmouth South MP, said she had visited his family earlier, describing them as "heartbroken".
On behalf of the family, she said she had spoken to the Foreign Office in a bid to find out how Mr Uzzaman died and if his remains could be brought back to Portsmouth.
She said she had been assured officials would look into it as a "top priority".
"Many people will have strong views about what Assad has done, but he was a much-loved son from a family who were completely integrated with British society with no extremist sympathies whatsoever," she said.
She also said she believed he and his friends "were well meaning when they went to Syria".
"The lesson here is simple: Anyone who thinks it a good idea to fight in Syria on any side should consider the very real possibility it will cost them their life," she added.
The final member of a group from Portsmouth who travelled to Syria to fight for the Islamic State (IS) group has reportedly been killed.