Nadal won in Barcelona between 2005-2009, 2011-13 and now in the last two years.
The 14-time Grand Slam champion looks like the man to beat in Paris as on the court newly baptised in his name he failed to drop a set all week.
Thiem had shocked world number one Andy Murray in Saturday's semi-finals and showed some early resistance.
The world number nine even had his only break point of the match in the first game.
But Nadal, returning to top form at age 30, upped his intensity at the crucial moment and took the set with the only break in the 10th game.
The second set was a procession as Thiem's frustration at his inability to break through Nadal's defences saw him consistently fire long.
Thiem held off three break points in his opening service game, but was powerless to stop Nadal thereafter as he won the last five games to seal victory in just over 90 minutes on court.