The British woman fighting the case is known to the court only as QT and was refused a spousal visa because she is in a same-sex partnership.
Major international companies including Morgan Stanley, Goldman Sachs and Credit Suisse last week made an application to submit evidence to the Court of Appeal, saying they wanted to give a "more rounded picture of the issues", emphasising their commitment to diversity and desire to hire top talent from around the world.
Human rights barrister Dinah Rose argued Thursday that even though gay marriage is not recognised under Hong Kong law, that did not mean same-sex couples in a civil partnership or married in a foreign jurisdiction should be denied the rights of heterosexual married couples.
Heterosexual spouses are eligible for "dependant" visas which allow them to enter Hong Kong and to work.
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Rose characterised such treatment as "a form of discrimination" that goes against the city's core values and the principle of equality.
QT came to Hong Kong in 2011 after she entered into a civil partnership with a woman known as "SS" in Britain.
SS was moving to Hong Kong for a new job.
But QT was denied a dependant visa and was given instead a visitor visa, which does not allow her to work.
She was later granted a 12-month visa under exceptional circumstances which did allow her to work, Rose said, but added that was still "less favourable" treatment.
Rose said that people in polygamous marriages are allowed to bring in one of their spouses as a dependant, even though such relationships are not recognised in Hong Kong.
Immigration law does not explicitly mention gay couples, but states that only the "spouse" of a person permitted to work in the territory may apply for a dependant visa.
Judge Andrew Cheung said Thursday gay people were treated "differently" in Hong Kong because of current marriage law.
"All I'm focusing on is in terms of the right to marriage."
Rights campaigners in the region were galvanised last month when a court in Taiwan ruled in favour of gay marriage.
The hearing continues and is due to last for two days.