Guimaraes, 24, is head coach Eddie Howe's third signing of the transfer window, following the arrivals of England international Kieran Trippier and New Zealand international Chris Wood.
"Bruno is a hugely exciting talent and has been one of our primary targets, so I'm delighted to secure him," said Howe.
"He strengthens the group immediately ready for the challenge ahead. He has been one of the standout performers in France and is a full international for Brazil." Newcastle have also reportedly agreed a fee with Brighton for defender Dan Burn, believed to be around £13 million .