They join three prisoners already refusing food, two months after mass hunger strikes in Israeli prisons ended in a deal expected to ameliorate treatment of Palestinian detainees.
Ayman Sharawna, 37, from Dura near Hebron, began refusing food on July 1 to demand his release from detention without charge, Addameer prisoners association said.
He had been released from Israeli custody under a prisoner swap agreement last October, but was rearrested on January 31.
Like four others recaptured by Israel after their release in the exchange deal, Sharawna is being held under a military order that allows him to remain in custody as long as a special committee is deliberating his case, after his amnesty under the exchange was canceled, Addameer says.
An Addameer lawyer who visited Sharawna earlier this week said he is being held in isolation in Ramon jail.
Meanwhile, the Palestinian prisoner society said sick prisoner Raed Gamal Ali Ahmad began a hunger strike on July 11 to protest what he describes as medical neglect by prison authorities.
Ahmad suffers from kidney disease, and says he has exhausted all possibilities in persuading prison officials to address his medical condition, according to the society, who visited him in Megiddo jail.
He has spent seven years in Israeli prison, after receiving a 16-year sentence.
Society head Qadura Fares said medical neglect in Israeli jail affects many prisoners who have health conditions.
Some 2,000 prisoners held an open hunger strike in April, before reaching a deal a month later which included allowing family visits for prisoners from the Gaza Strip and moving prisoners out of solitary confinement.
Prisoners say Israeli authorities have not kept all its terms, including renewing the administrative detention orders for prisoners and failing to improve living conditions.
One prisoner from the Gaza Strip has been on hunger strike for 97 days seeking an early release due to illness.
A diabetic, Akram al-Rekhawi, 39, has been held in a prison clinic since he was detained by Israeli forces in June 2004. He has served eight years of a nine-year sentence.
Another detainee, Samer Al-Barq, 36, is on his 57th day of hunger strike after his administrative detention was renewed.
Administrative detainee Hassan Safadi, 34, is on his 27th day of renewed hunger strike, after his detention without charge was renewed in violation of the agreement ending the hunger strikes in May, Addameer says. He was previously on hunger strike for 71 days.