Yekaterina Novikova said Friday Filin had third-degree burns to his face and eyes and was being treated at a Moscow hospital where doctors tried to save his sight, RIA Novosti said.
Novikova said Filin's recovery could take at least six months.
"Sergei is in a satisfactory condition," a hospital representative said. "He is in our burns center, not intensive care."
The attack occurred late Thursday as Filin was walking home, the BBC said.
No arrests had been made and a motive was unknown but investigators said they believe it could be traced to internal theater politics.
Filin, 43, a former Bolshoi Ballet star, beat out several others to become the ballet's artistic director in March 2011. Several dancers have resigned in protest over its new program, the BBC said.
The acid attack followed several months of intimidation, during which Filin received threatening phone calls and the tires on his car were slashed, RIA Novosti said. Shortly before the attack, an attempt was made to hack his Facebook page.