She and James brought new baby Louis home in January.
The couple began the adoption process four years back but managed to keep his arrival secret.
However, the Oscar-winning-actress has told People magazine's website she is now adopting the three and a half-month-old boy, who was born in New Orleans, alone.
'Yes, I have filed for divorce... I'm sad and I am scared,' the Mirror quoted her as telling the mag.
A rep for Bullock confirmed to the mag that the star had filed legal papers seeking to end her marriage.
Image: Sandra Bullock with Jesse James during happier times.
Photograph: Danny Moloshok/Reuters
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