Anne McClain acknowledges accessing the account from the ISS but denies any wrongdoing, the New York Times reports.
Her estranged spouse, Summer Worden, reportedly filed a complaint with the Federal Trade Commission.
Ms McClain has since returned to Earth.
The astronaut told the New York Times through a lawyer that she was merely making sure that the family’s finances were in order and there was enough money to pay bills and care for Ms Worden’s son – who they had been raising together prior to the split.
“She strenuously denies that she did anything improper,” said her lawyer, Rusty Hardin, adding that Ms McClain was “totally co-operating”.
Ms McClain and Ms Worden, who is an Air Force intelligence officer, married in 2014 and Ms Worden filed for divorce in 2018. Investigators from Nasa’s Office of Inspector General have contacted both over the allegation, the New York Times reported.
Ms McClain graduated from the prestigious West Point military academy and flew more than 800 combat hours over Iraq as an Army pilot. She went on to qualify as a test pilot and was chosen to fly for Nasa in 2013.
She spent six months aboard the ISS and had been due to feature in the first all-female spacewalk, but her role was cancelled at the last minute over what Nasa said was a problem with availability of correct suit sizes.
How does the law work in space?
Legal frameworks agreed by the five states that own the space station – the US, Russia, Europe, Japan and Canada – set out that national law applies to people and possessions in space.
So if a Canadian national were to commit a crime in space, they would be subject to Canadian law, and a Russian citizen to Russian law. Europe exists as one state within the legal framework, but any of the European states may extend their respective national laws and regulations to the European equipment and personnel in space.
Space law also sets out provisions for extradition back on Earth, should a nation decide it wishes to prosecute a citizen of another nation for misconduct in space.
As space tourism becomes a reality, so might the need to prosecute space crime, but for now the legal framework remains untested. Nasa officials told the New York Times that they were not aware of any crimes committed on the space station.