Damian Jennings' team never looked back after a 17-4 first period, and a good all-round team performance saw them take the win and a place in the top flight.
Mairi Buchan led the scoring with ten points, and added eight rebounds, while Katrin Chiemeka hit three of her five three-point attempts to finish with nine points, and also added eight boards and three steals.
Mary Durojaye also scored nine points and as many rebounds, while Kristie Sheils finished with seven points and four assists.
After that blistering start, GB remained in control and were 28-15 ahead at half-time having shared 22 second period points.
The Portugese - who were led by 11 points and as many rebounds from Sofia Da Silva but had no one else in double figures - shaded the third period12-11 to cut GB's lead to 38-27 at the final break.
But they had no answer to the GB team in the final stanza as they outscored them 20-13 to take the victory.
A 52-37 success rate on the boards made up for a poor night shooting, as Jennings and his team were left to celebrate.
