A US judge said the plaintiffs could pursue claims that the film unit of the Sony corporation had been negligent.
The plaintiffs are seeking to hold Sony liable for not bolstering security after previous breaches.
"We are pleased that the court has properly recognised the harm to Sony's employees," said lawyer Michael Sobol.
Many Sony employees had their personal details made public in 2014 when a group calling itself Guardians of Peace leaked data from the studio's computers.
The attack was described as an act of revenge motivated by Sony's release of The Interview, a comedy about an assassination attempt on North Korea's leader.
The nine plaintiffs claim Sony Pictures Entertainment violated a California confidentiality law by spurning security measures to stop the theft of employees' salary and health data, Social Security numbers and other sensitive information.
Without ruling on their action's merits, US District Judge Gary Klausner said Sony had created a "special relationship" with its employees by requiring them to provide personal information to be eligible for salaries and benefits.
The former workers said Sony's negligence caused them economic harm and that the hack had been "an epic nightmare, much better suited to a cinematic thriller than to real life".
The Interview, starring James Franco and Seth Rogen as journalists recruited by the CIA to assassinate Kim Jong-un, was initially withdrawn from cinemas but was later made available online.

Contact with flight MH370, carrying 239 people, was lost en route from Kuala Lumpur to Beijing. Despite an extensive search no trace has ever been found.
The trial system enables planes to be tracked every 15 minutes, an increase on the current 30 to 40 minutes.
It uses technology already installed on most long-haul jets.
The system is expected to increase the tracking rate to five minutes or less if there is any deviation from a plane's expected route.
Australian Transport Minister Warren Truss said the new system was a "world first". But he stressed the new technology would not necessarily have solved the mystery of MH370.
"It would have been very difficult, one would imagine, without knowing what precisely occurred in the case of MH370, to have intervened from outside," he said.
"But at least it would have tracked the aircraft to within 15 minutes."
Airservices Australia Chairman Angus Houston, who helped lead the search for MH370, agreed it was "no silver bullet".
"But it is an important step in delivering immediate improvements to the way we currently track aircraft while more comprehensive solutions are developed," he added.
The trial will begin in the Australian city of Brisbane, before being extended to Indonesia and Malaysia.
Investigators searching for MH370 are focusing on an area of the Indian Ocean off the coast of western Australia.

A trailer and tractor backed into Harry Whitlam at Swithens Farm, Rothwell, West Yorkshire, on 9 August 2013. He died later the same day.
Leeds Coroners' Court heard Harry was struck at an area of the farm not usually open to the public.
Tractor driver Gary Green was arrested but never charged.
Mr Green had 90 micrograms of alcohol in 100ml of breath at the time of the accident. The legal limit is 35 micrograms.
The court heard Mr Green was arrested on suspicion of drink-driving and causing death by dangerous driving but was later released and never charged.
West Yorkshire Police said a criminal prosecution could not be brought because the incident occurred on private land.
Jurors were told Harry had gone into the area where he was run over just after 09:00 BST.
His mother, Pamela Whitlam, who worked at the farm's cafe, said she had not explained to him where he could or could not go because she was not familiar with the layout of the site.
In a statement to the court, she described how just before the accident she told her son: "Be careful."
Moments later she ran outside to find him lying on the floor after being hit by the vehicle.
Jurors heard Mr Green had drunk "three of four pints" at the pub the night before the accident and continued drinking at home until 01:45.
Following his arrest, he told police officers: "I don't know where he came from because no-one should be down the back of the tractor."
The inquest continues.
Update 31 July 2015: This story has been amended following updated information supplied by West Yorkshire Police about why a prosecution could not be brought.
Sony Pictures Entertainment has failed in an attempt to dismiss a legal action brought by nine ex-employees over the cyber hack that crippled the studio.
[NEXT_CONCEPT]
Australia, Indonesia and Malaysia are to trial a new method of tracking planes, almost a year after a Malaysia Airlines flight disappeared.
[NEXT_CONCEPT]
A tractor driver was almost three times the legal alcohol limit when he reversed into an 11-year-old boy, an inquest has heard.