The Dutchwoman improves to 12-1 on clay this season and will face 10th seed Petra Kvitova in the championship match.
The 20th-ranked Bertens continues her impressive run in Madrid after prior victories over world number two Caroline Wozniacki in the third round and Maria Sharapova in the quarterfinals.
She'll aim to win her sixth career title, all on clay, including a Premier win on the green clay of Charleston this year.
Bertens pulls away to take first set
Bertens struck the first blow in the third game. Garcia's forehand was sprayed wide to set up break point at 30-40 and she committed another careless error, her backhand flying wide to hand the Dutchwoman a 2-1 lead.
After both players recorded battling holds, Bertens ran off the final three games of the set. After a hold to 30 for a 4-2 edge, the world number 20 effectively put the set to bed.
The world number seven continued her erratic play off the ground, committing back-to-back forehand errors to gift Bertens a second break and a 5-2 lead. The Dutchwoman held to 15 to close out the opening set in 31 minutes.
Dutchwoman breezes through second set to reach final
In a virtual replay of the beginning of the first set, Bertens was able to gain the early edge in the third game. A smart slice forced a volley error from Garcia to set up a break point and the Frenchwoman's drop shot just missed to give the Dutchwoman a 2-1 lead.
After digging out of 15-30 to hold, Bertens again displayed her outstanding defensive skills to set up another break chance, duly taken when the world number seven missed a backhand volley, the world number 20 in cruise control with a double break lead at 4-1.
Serving for the match at 5-2, an example of the outstanding level of tennis being played by Bertens came at 15-15 when she bulleted a backhand down the line after withstanding several Garcia forehands, turning defense into offense.
At 30-30, an ace and a final forehand error from the Frenchwoman sent Bertens to her second Premier final of the year on clay.
Bertens "happy" with the way she played, Garcia frustrated
Following the match, Bertens said: “I'm really happy with the way I played. I think it was maybe not great tennis, what people want to see with a lot of winners, but Caroline is a great player.”
She also said mixing it up against Garcia was a key to her success: “She likes when the ball comes from the hip so I was just trying to play as many different balls as I could. I think I did it pretty well.”
Garcia agreed: “She likes when the ball comes from the hip so I was just trying to play as many different balls as I could. I think I did it pretty well.”
The Frenchwoman admitted “I couldn’t do anything about it. I tried different things, but it didn’t work. Of course, I’m frustrated."
While she conceded: "My performance today was not great", (she committed 24 unforced errors), she also praised her opponent, stating "she had a game plan and it worked well.”