Campaigners said the case is a landmark for the socially conservative southern Chinese city, which does not recognise gay marriage and only decriminalised homosexuality in 1991.
QT, as she is referred to in court, entered into a civil partnership in Britain in 2011 and moved to Hong Kong the same year after her partner was offered a job in the city.
However, after being denied a dependant visa, she is currently on a tourist visa and is not permitted to work.
"All I want is equal treatment," she told AFP. "If our (heterosexual) friends can easily get a dependant visa then why can't we? I don't want to be treated as a second-class citizen," she said.
The former British colony does not explicitly rule out gay couples, but its immigration law states that only the
Heterosexual couples who are not married are not eligible for the visa either.
QT and her partner are not legally married, although gay marriage was legalised in England and Wales in 2013.
Lawyer Michael Vidler, who is helping QT mount her case, said it is "unconstitutional" for the government not to recognise civil partnerships conducted overseas.
"They are being unreasonable in the policy they are taking, misapplying the law and the concept of spouse according to the law," he said, adding that QT had been told by Hong Kong authorities that the city does not recognise same-sex couples.
The immigration department declined to comment when contacted by AFP about the case, which is expected to be heard over two days.
"The impact of the case will be big. It will affect a lot of people who wish to come to Hong Kong and whether Hong Kong can maintain our position as a place to do business," gay rights activist Billy Leung told AFP.
Last June Hong Kong refused to allow same-sex couples to marry at the British consulate in the city, prompting heavy criticism from gay and lesbian rights groups.
The decision came after the British Foreign and Commonwealth Office announced it would allow its overseas missions to perform same-sex marriages for Britons and their partners in countries where it is illegal under local laws.