Frankie Boyle says he has not eaten for four days in a bid to highlight the case of Shaker Aamer - the last UK resident being held at Guantanamo Bay.
The Scottish comedian has joined campaigners who are attempting to fast for a combined total of 1,000 hours.
Mr Aamer, from London, has been detained in the military prison for 11 years without being charged or tried.
Since February, 100 of the 166 prisoners still held have been refusing food in protest at their detention.
Human rights campaigners Reprieve are supporting the hunger-strikers by encouraging supporters to give up food temporarily.
'Like being drunk'
Clive Stafford-Smith, a lawyer and the charity's legal director, is one of those fasting.
"Frankie's action is more important than mine," he wrote.
He added he wanted the "symbolic" campaign to provoke debate and pressure US President Barack Obama to make good on his promise to close the camp.
Boyle, normally known for his acerbic and often controversial brand of humour, told his 1.4m Twitter followers he was on the fourth day of his hunger strike.
On Thursday he tweeted: "Day two of hunger strike feels a bit like being drunk. Feel pretty good, but no doubt I'll wake up to find myself in bathroom eating soap."
He has also been promoting Reprieve and publishing links to campaign sites calling for Mr Aamer to be freed.
The 47-year-old, whose wife and children still live in south-west London, has been cleared for release.
He told the BBC in May he was losing his mind, health and life in Guantanamo - the military prison used by the United States to detain al-Qaeda and Taliban prisoners since 2002.
US authorities said he led a unit of Taliban fighters against Nato troops and had met Osama Bin Laden. But Mr Aamer has always said he was in Afghanistan with his family doing charity work.
At least 14 British nationals and residents were held at Guantanamo Bay at one time, with Binyam Mohamed's release in 2009 leaving only Mr Aamer there.
The UK government has said it is continuing to press the US to get him released.