Tewell shot 2-under 70 to finish the tournament at 11-under 205, one stroke better than Bobby Wadkins, who made a late run with a 66 in the final round.
Stewart Ginn shot 72 to tie with Bob Gilder (70) for third at 8-under.
Tewell, 52, won for the second time this season, joining Tom Kite and Hale Irwin as multiple winners this season.
Tewell has two senior major championships to his name and is the most accurate driver on the Senior Tour (86 percent). He only improved the numbers over three days on The King & The Bear course at World Golf Village.
He hit all 14 fairways Sunday and finished the tournament hitting 41 of 42. The only one he missed came Friday at No. 7, when he hooked a drive to the left.
'It was in the first cut of rough,' Tewell recalled. 'No big deal.'
In the final round, Tewell trailed Ginn by one stroke heading into the par-5 13th hole.
But the lead flip-flopped when Tewell made a 10-footer for birdie, and Ginn missed from 8 feet to save par, a putt set up when he overcooked his approach on the second shot and had to make a tough shot to get on the green.
Ginn, who played with Tewell on both Saturday and Sunday, said he never got discouraged, even though Tewell didn't miss a fairway either day.
'I knew what I had to do to beat him,' Ginn said. 'I went for it. I didn't back off. If he makes birdies, he makes birdies. You've got to go out there and golf your golf ball.'
Proving how much easier this game is when you're constantly playing from the fairway, Tewell also hit 47 of 54 greens in regulation over the three days.
'That's my golf game,' he said. 'I try not to make a lot of mistakes. I don't make a lot of mistakes. That's why when I get in the lead, sometimes I'm able to go ahead and win big.'
Although this wasn't a big victory, it was a consistent performance, and the result wasn't really in doubt as Tewell made his way through the final four holes.
On the par-3 14th, he put his tee shot 18 inches from the hole for birdie to go to 11-under and take a two-stroke lead.
Wadkins, playing three groups ahead, made birdie on No. 18 to pull within one, but Tewell finished with four straight pars. When he tapped in on No. 18 for the championship, he doffed his cap and waved to the small crowd to celebrate his sixth career victory on the Senior Tour.
Tewell earned $306,000, bringing his career earnings on tour to $3.99 million.
'I knew Doug wasn't going to play a sloppy round, so I just went out and tried to improve my position,' Wadkins said.
Also in the hunt was Gary McCord, who was one stroke behind the leaders until he knocked his second shot off the rocks and into the water on the par-4 15th. He made bogey on his way to a 70 and a fifth-place finish at 7-under.
'Greedy, greedy, greedy got me right there,' McCord said of the shot on No. 15. 'I hit it hard, but slid under it a little bit. It's just a bad shot.'
News, Notes and Numbers
*Irwin blew his chance for a win with a double-bogey on No. 12 that pushed him from three strokes of the lead to five. He finished tied for sixth.
*Proving golf isn't always pretty, Fuzzy Zoeller nearly fell on his backside after blasting out of a bunker from an awkward stance on the 10th hole. He got big applause from a curious gallery, which had to rubberneck to watch the shot from across the 12th fairway.
*Officials are considering several options that would relocate the Legends of Golf, the tournament credited with spawning the Senior Tour. This was the fourth straight year the tournament has been played at the World Golf Village, and attendance - while not officially listed - has been light.
Full-field scores from the Liberty Mutual Legends of Golf