According to US Weekly, the 30-year-old former girlfriend of Jude Law gave birth in London over the weekend.
Sienna and the baby's father - her 26-year-old fiance Tom Sturridge - have made no comment about the reported arrival, and details of the baby's sex, weight and name are not known.
Sienna was tight-lipped about her pregnancy for several months and refused to comment when news of it broke back in January, but eventually spoke about it to British Vogue in April, admitting she was 'under strict instructions' not to discuss it.
Doing so, she said would 'completely defeat the point of everything I've tried to achieve in the last eight years'. The yummy-mummy-to-be did say though that she was 'feeling fine' and that it was 'all progressing nicely and it's very exciting.'
She and The Boat That Rocked star Tom became engaged in February.
Aw! What exciting news - we shall eagerly await more details!