Sunita Williams is a flight engineer aboard the space station
As of 0547 GMT on Saturday, she had passed the 188-day, four-hour mark set by Shannon Lucid in 1996.
Mrs Williams, who is of Indian and Slovenian extraction, is due to return to Earth on Thursday on the Atlantis.
She earlier set a record for the longest spacewalk by a woman, reaching 22 hours and 27 minutes.
That record had been held by astronaut Kathryn Thornton at just over 21 hours.
In her six months in space, Mrs Williams has also "run" the Boston Marathon on a treadmill.
Sunita Williams


United States astronaut , currently on board the International Space Station, has today broken the record for the longest unbroken space flight by a woman. At 5:47 a.m. UTC Williams passed the previous record of 188 days and four hours set by  in 1996.

This is not the only record Williams has broken. Earlier this year she set a record of 29 hours and 17 minutes for time spent by a woman on space walks. 

In April 2007, Williams received much attention when she took part in the Boston Marathon on a treadmill whilst in orbit.