The local coast guard launched a search and rescue operation after the alarm was raised for the Chinese national in her 20s in the central Shizuoka region on Monday evening.
"It was around 7:55 pm on July 8 when we received the information after the woman's friend reported to a nearby convenience store that she was missing," a local Japan Coast Guard official told AFP.
The woman, who was not named, told rescuers that she was swept out to sea and could not return to the beach as she was swimming with a rubber ring.
She was finally spotted floating off the southern tip of Chiba's Boso peninsula at 7:48 am on Wednesday by a cargo ship, the official said.
Two crew members of a smaller nearby tanker contacted by radio jumped into the sea to rescue her.
"There are 80 kilometres in a straight line (between the beach and rescue spot)... but it is assumed she drifted for an even greater distance," the official said.
The woman was taken to hospital after the rescue but did not need to be admitted as she was clear-headed and her dehydration was not life-threatening.
