Helped by Isner's wayward shot-making, Ferrer rallied from a first-set deficit to stay unbeaten on clay in the competition with his 16th straight win.
The fifth-ranked Spaniard scored six break points and his American opponent was left slamming his racket in frustration towards the end as 70 unforced errors undid a positive start.
Spain, which will try to win its fourth title in five years, stretched its record home winning streak to 24 straight ties. It hasn't lost on clay since 1999.
Spain will play at the Czech Republic or Argentina in November's final.
Ferrer showed again he could fill the leadership void left by Rafael Nadal, who was injured and had also said he would scale back his Davis Cup availability after clinching last year's final triumph against Argentina.
"I'm very happy and this is a dream for me. Another Davis Cup final," said Ferrer, who has played a part in each of Spain's last three titles. "For me (it's) the best emotions, Davis Cup. I really want to enjoy this moment now."
"It's been a wonderful year and now we want to go for more, of course," Corretja said. "(Ferrer's) a guy who never gives up, he's a gladiator and he knows the way he needs to play on clay, the way he needs to move and hit the ball."
Isner helped too, by hitting more than twice as many unforced errors as Ferrer in the near capacity 15,000 seat venue on the Asturian coast to look nothing like the player who beat Roger Federer in Switzerland in the competition this year.
The American started well, ripping a number of his 28 forehand winners past a tentative Ferrer who stuck to his baseline, and a pair of his 16 aces helped secure the lead when Ferrer's forehand sailed long in the first-set tiebreak.
Isner's momentum came undone quickly, however, as his forehand became wayward to help his opponent, who saved four of the six break points he faced, draw even and find his rhythm.
Isner was coming apart as his serve lost pace - it clocked 229 kph (143 mph) in the first set - as he dug himself into 0-40 spot in the seventh game of the third set. The 10th-ranked American hit a forehand long to be broken and, later, dropped his racket to the ground and kicked it before Ferrer took a 2-sets-to-1 lead.
Isner's frustration carried into the fourth set as he threw his racket and kicked it again as Ferrer broke for 2-1.
When Ferrer hit a passing winner in the fifth set, Isner's expression was of resignation as he netted an easy forehand to be broken again. His backhand flew wide to clinch Spain's 26th straight victory on clay.