Salon Meritage opened its doors Sunday after undergoing months of extensive remodeling.
Acosta is an employee who escaped the massacre in October 2011. She had stepped out for a break when the shooting occurred.
The incident claimed the lives of eight people, and left one badly injured.
Police say that the alleged gunman, 43-year-old Scott Dekraai, was enraged over a custody battle and targeted the hair salon where his ex-wife worked.
Dekraai first shot his ex-wife, Michelle Fournier, and then her friend and co-worker Christy Lynn Wilson, according to witnesses and grand jury transcripts.
Salon owner Randy Fannin became the third victim when he tried to intervene and coax Dekraai outside.
Prosecutors say Dekraai then worked his way through the business, shooting employees Victoria Buzzo, 54; Michele Daschback Fast, 47; and Laura Webb, 46.
Customers Bernice Kondas, 65; and Harriet "Hattie" Stretz, 73, were also shot. Stretz survived her serious injuries.
Then, as Dekraai walked out of the shop, he is accused of gunning down David Caouette, 64, who was sitting in his Range Rover.
According to court transcripts, Dekraai shot Caouette because he thought he was an undercover police officer.
Salon co-owner Sandy Fannin, who lost her husband in the shooting rampage, spoke at a memorial held on the one year anniversary last month.
"After living through this horrible tragedy that occurred last October 12, I didn't think that I would be able to step back into the salon again," she said.
"But with the strength of my Lord Jesus Christ, the love and support of my family, friends and our community, the Salon Meritage will re-open."
"You guys have helped give us the strength to keep going," said Paul Caouette, son of victim David Caouette.
"You will always be in my heart and in my soul," said Paul Wilson, the husband of Christy Lynn Wilson. "I won't ever let anyone forget you."
Dekraai was indicted on eight counts of murder and one count of attempted murder on Jan. 17.
According to friends and family, Fournier lived in fear of her former husband, with whom she was involved in a bitter custody battle over son Dominic, who is now 9.
Court records show that Dekraai had accused Fournier of being an unfit parent.
She, in turn, charged that Dekraai's mental state had deteriorated after the loss of a leg in a tugboat accident that killed a co-worker.
Dekraai was also diagnosed as bipolar and required heavy medication, according to court documents.
The couple was in an Orange County Family court just the day before the shooting, with Dekraai's claims being rejected, O.C. District Attorney Tony Rackauckas said.
Their son is now being cared for by elder step-sister, Chelsea Huff, 25, who shares guardianship with Fournier's parents.
He is said to be getting involved with sports and doing well.
"They're just doing great," Chelsea's father, Steven Huff, said. "Chelsea's dedicated to her little brother with, you know, the support of her family."
The trial for Dekraai is expected to begin on March 25, 2013.