Simon Cowell is officially a dad!!!!!!

Simon Cowell is officially a father ... his pregnant GF Lauren Silverman has finally popped in NYC.

36-year-old Silverman just gave birth to a baby boy at Lenox Hill Hospital in NYC -- after Simon flew across the ocean from London to be by her side for the delivery. As we reported, Lauren went into labor early this morning and Simon hopped a plane as soon as he heard.

The boy is 54-year-old Simon's first child and Lauren's second -- she also has an 8-year-old son with her ex Andrew Silverman.

So far, no word on the baby's name.