Stress from working on a book brought on Samphan's high blood pressure, the tribunal spokesman said.
His hospitalisation came as the tribunal was hearing an appeal by Ieng Thirith, another former senior Khmer Rouge leader, against her continued pre-trial detention.
Ieng Thirith was the sister-in-law of Pol Pot, the Khmer Rouge's deceased supreme leader.
She and Khieu Samphan are among five suspects facing trial for their alleged roles in the death of up to two million Cambodian's during the Khmer Rouge rule over the country in the 1970s.
Known as the "first lady", the former social affairs minister was arrested last November along with her husband Ieng Sary, the regime's foreign minister.
The Cambodian lawyer for the 76-year-old Ieng Thirith has cited a lack of evidence for detaining her and said she suffers from chronic illnesses.
However, court officials say doctors have deemed her fit to stand trial.
The first formal trials under the much-delayed tribunal are expected to begin later this year.
However, critics have said the process is fast running out of time, with the few surviving former Khmer Rouge leaders in poor health.