The Israeli government confirmed the death to the Thai embassy in Tel Aviv on Wednesday, although it was not clear when the latest victim had died.
The number of injured remained unchanged at 16 while 18 are presumed to have been taken hostage by Hamas.
Srettha was speaking to reporters in Beijing on Wednesday morning.
He met UN Secretary-General António Guterres on the sidelines of the 3rd Belt and Road Initiative forum.
Srettha said he had told the UN chief that despite Thailand not being a party to the conflicts over Palestine, it had suffered the second highest death toll after the US.
Srettha said the UN secretary-general was shocked by the loss of Thai lives and expressed his condolences.
Srettha said the UN chief told him that he would travel to Egypt and visit the area near the Gaza Strip to pressure for an end to the war as soon as possible.
On the repatriation of Thai workers from Israel, Srettha said the government could repatriate about 400 Thais per day and would soon be able to increase the capacity to 600 a day.
The prime minister added that the Thai government was not complacent about the safety of the remaining Thai workers in Israel.
He said Foreign Minister Parnpree Bahiddha-Nukara was in the process of seeking permission for the use of ports of Middle East nations to repatriate Thais by sea.
Srettha said the US has repatriated its 1,500 to 2,000 citizens by boat to Cyprus but the Israeli ports have been closed.
The prime minister added that many Thais were still unable to leave the risky zones in Israel and dozens of Thais could not reach the airport in time for their flights although they had registered with the Thai embassy for repatriation.
He said the Supreme Commander of the Thai Armed Forces had been trying to coordinate with the Israel Defence Forces, seeking help to evacuate Thais from risky zones as soon as possible.
Srrettha also urged the remaining Thais to decide as soon as possible whether they want to continue working in Israel at their own risk or return to Thailand.
“The risk is on you. The government has a duty to repatriate people fast and safe after you make a decision to return home,” Srettha said.