Frost and Langer each shot 6-under 66 to finish at 8-under 208 on TPC San Antonio's Canyons Course.
"I wasn't planning on this," Frost said. "Only when I hit it close on 13 (to set up birdie) did I think that it was just Bernhard and I (with a chance to win)."
Second-round leader Mark Calcavecchia had a 74 to finish two strokes back. He bogeyed four of the first 11 holes.
"I putted bad on the front," Calcavecchia said. "I didn't hit it very good. I knew it yesterday, when I hit those bad drives on two holes (late in the round). I didn't feel comfortable."
The 53-year-old Frost also teamed with Michael Allen to win the Legends of Golf in April and has three career Champions Tour victories. The South African won 10 times on the PGA Tour.
He made a 30-foot putt to save par on the 14th hole.
"He had some unbelievable up-and-downs today," Langer said. "The par he made on 14 was unbelievable. He had three or four other ones. The putter was hot. He hit a few loose shots but always got away with it."
On the first extra hole, Langer lipped out a 10-foot birdie putt. That gave Frost the opportunity, and he cashed in with a 10-foot putt to seal the biggest comeback in the 26-year history of the event.
Langer, the German star who has two victories this year, left his approach in regulation on No. 18 short in a deep bunker. His blast went about 6 feet past, and made the putt to save par.
Frost missed a birdie putt of about 25 feet and tapped in for par to set up the playoff.
Tommy Armour III shot a 67 to finish fourth at 5 under.
The tournament was the final full-field end of the year. The top 30 on the money list qualified for the Charles Schwab Cup Championship at Desert Mountain in Scottsdale, Ariz.
The top 30 remained unchanged, but it did get interesting.
Tom Kite entered the week 31st, $32,506 behind Mark McNulty for 30th. With a 13th-place finish this week Kite cut that margin to $3,831 but remained behind McNulty and will miss the season-ending event for the second straight year.
Langer came to San Antonio with the lead in the Charles Schwab Cup standings and added another 100 points to his cushion. He leads Tom Lehman by 211 points.
Three players -- Langer, Lehman and Roger Chapman -- can top the standings in Scottsdale. Lehman won the $1 million annuity last year.
"It all depends on the scenario next week," Langer said, "and there are lots of scenarios. I haven't done the math."