Italian astronaut Luca Parmitano was on his second career spacewalk when he reported feeling water pooling at the back of his head.
Soon, he was unable to see, hear or talk and was rushed back into the space station, where he quickly recovered.
The US space agency said it has never seen a problem like this before, and it appeared that the source was unlikely to be the drink bag that astronauts carry inside their helmets.
Another possibility was that the fluid came from the spacesuit's cooling system, NASA said.
"Clearly we have a problem at this point that we don't quite understand," said Kenny Todd, mission management team chairman.
Drowning or suffocating were potential risks, had the situation gone on any longer, NASA experts told reporters.
It also caused momentary alarm for Parmitano, an astronaut with the European Space Agency.
"Go stick your head in a fishbowl and try to walk around," said David Korth, NASA flight director, describing the discomfort that the novice spacewalker felt as he grappled with the leak.
"He was clearly having trouble," said Korth, adding that the 36-year-old "stayed calm and cool" even as the amount of water appeared to increase as he made his way back to the airlock.
As crew members helped remove Parmitano's headgear, globs of water could be seen floating away.
Shortly afterward, he described for his fellow astronauts a strange liquid that appeared to come from inside the back of his helmet.
"Luca says the water tastes really funny," US astronaut and fellow spacewalker Chris Cassidy told NASA mission control in Houston.
"To him the water clearly did not taste like our normal drinking water," said Cassidy.
Parmitano's long underwear inside the suit was dry around his midsection, and it appeared the leak had come from the vent port near the back of Parmitano's helmet, Cassidy added.
Cassidy said his colleague looked "miserable" but was doing "OK," and NASA TV showed images of Parmitano floating inside the space station and blowing his nose with a tissue.
Karina Eversley, lead spacewalk officer, said the drink bag was "unlikely" to be the source of the leak.
NASA is probing whether the leak may have come from the liquid cooling ventilation system in the spacesuit, which contains about a gallon of water treated with iodine and may have a bad taste like Parmitano described.
"We have not seen a problem before with this type or quantity of water," she said.
Parmitano on July 9th became the first Italian to walk in space. He was wearing the same spacesuit as he wore during his inaugural outing.
The purpose of the spacewalk, the second of two planned this month, was to prepare the ISS for a new Russian module and to make some repairs.
Parmitano had finished his first task of making a cable connection outside the ISS when he reported a faulty CO2 sensor. About six minutes later he began feeling a "gush" or a "bubble" at the back of his head, Korth said.
The outing was cut short at one hour 32 minutes. It was supposed to last six hours and 15 minutes.
The spacewalk was the second shortest in history, following a 14-minute outing in 2004 that was terminated early due to a pressure sensor failure in the Russian-made suits, a NASA spokesman said.
Parmitano arrived at the ISS aboard a Soyuz on May 28th for a six-month mission, joining Cassidy and two Russians who arrived in March.