Park wardens said global warming might be responsible.
The Patagonian glacier known as the "White Giant" is one of Argentina's biggest attractions. The river of ice 18 miles (30 km) long ends in a sheer wall blocking Lago Argentino where large pieces tumble into the water from time to time.
Tourists and locals visited the site in recent days, hoping to catch a glimpse of the rare spectacle, but only a few were on the observation deck when the roof of an ice tunnel caved in early on Wednesday, a National Glaciers Park official said.
"It's the first time the glacier's broken in winter (since records began)," park warden Carlos Corvalan said earlier this week, when the glacier started to crack.
Wednesday's rupture occurred after several days of partial break-ups, according to the provincial government's Perito Moreno Web site, www.lupacorp.com/glaciar/us/.
The glacier sheds ice roughly every four years, and the last time big ice chunks fell off was March 2006.
Argentina's Glaciers Park is home to more than 200 glaciers and is the biggest continental ice extension in the world after Antarctica, according to the park's Web site.
(Reporting by Helen Popper and Walter Bianchi, Editing by Anthony Boadle)