The Potter author gave birth on Sunday afternoon in an Edinburgh hospital, but news of the birth only came out on Monday night.
And the baby's name? It's David Gordon Rowling Murray - a good old Scottish name.
The proud mother and baby are "doing well", said JK's spokesperson.
The much-loved Potter author and her son were allowed home to rest.
It means a little baby brother for Jessica, who's nine.
JK - who's already started work on Potter book six - is set for a big year with the new baby as well as her long-awaited Order of the Phoenix due out on 21 June.
And we bet David will get the best bed-time stories!