Omar Abu Shalal, 55, of Nablus, West Bank, was moved to Israel's Ramla prison hospital, Prisoner Society lawyer Jawwad Boulous told the Palestinian Ma'an news agency Wednesday.
Boulous said Abu Shalal, who has refused food and drink, is suffering stomach pains, poor vision and severe headaches.
Abu Shalal is in the same prison hospital room as Bilal Diab and Thaer Halahla, who have been on a hunger strike for 36 days, Boulous said.
In a letter they gave Boulous, the three detainees said, "We stand for those who support us and we call for freedom and we consider your support as food which gives us strength and determination."
Hunger strikes have focused international attention on Israel's policy of detaining Palestinians without charge. Ma'an said thousands of Palestinians have been held by Israel without charges since 1967.
The news agency said about 300 Palestinians now being held in administrative detention in Israel are not told why they're held and aren't tried in court. Their lawyers aren't informed of any evidence against them, Ma'an said.