After 11 detainees joined the protest overnight, there are now 131 taking part in the hunger strike, said Major Jeff Weir, a spokesman for the detention centre on Thursday.
Twenty-one of the striking prisoners were hospitalised in stable condition at the prison medical clinic, including 20 who were being tube-fed, Weir said.
This is the latest in a series of hunger strikes since 2002 by the detainees. Many of the inmates have been held without charge for more than three years and some prisoners have threatened to starve to death unless they are put on trial or released.
The military considers a hunger strike to be a form of suicide and Weir said that one prisoner was restrained after he resisted having a feeding tube inserted.
"No facility in the US, and hopefully the world, is going to let someone starve to death," Weir said. "We're charged with keeping them in good health, and that's what we're doing."
Guantanamo officials said this latest hunger strike began on 8 August with 76 detainees protesting their confinement.
It has since grown to more than a quarter of the approximately 500 detainees.
"We're charged with keeping them in good health, and that's what we're doing"
Major Jeff Weir,
Spokesman at Guantanamo Bay
The military defines someone as being on a hunger strike after they have refused nine consecutive meals. There have been at least two previous hunger strikes, but military officials said they were not able to confirm whether this was the largest.
A US human rights group, the New York-based Centre for Constitutional Rights, has claimed that some 210 detainees are participating in the hunger strike.
The detainees are accused of ties to the al-Qaida terror network or Afghanistan's ousted Taliban regime. Most have been held for more than three years without charge.