'Last week I felt like I played well enough to win, but you always doubt that you'll win again,' Tambellini said. 'This takes a lot of pressure off. I don't want to say I've achieved my goal, but I'm a lot closer to it.'
Troy Matteson, who held at least a share of the lead after the second and third rounds, managed a 3-under 69, but it was only good enough to share second place. He was joined at minus-20 by Vance Veazey (67), Steven Bowditch (64) and David Peoples, who closed with a 10-under 62.
Jason Dufner and first-round co-leader Bill Haas were one stroke further back at 19-under-par 269.
Tambellini got his round going with a birdie on the second at Highland Springs Country Club. After five straight pars, he birdied the par-5 eighth for the fourth straight round to get to minus-16.
The 30-year-old birdied the 11th and made it two straight with a birdie on the 12th. Tambellini, who won the 2003 Boise Open, dropped in his fifth birdie at the 15th to get to 19-under.
Tambellini made it two in a row as he chipped in for birdie on the 16th. That birdie gave him a share of the lead at 20 under. He closed with a two-putt birdie on the par-5 last to earn his second tour win.
'I hit a perfect putt, or so I thought,' said Tambellini of his putt at the last. 'That last one was a nervous putt. I definitely took my time on it.'
Matteson, who led by four shots after two rounds, birdied the first to extend his early lead. However, he could only par his next eight holes. Around the turn, he dropped a shot at the 10th.
The Virginia Beach Open winner dropped in birdies at 11 and 12 to get back to minus-19. Matteson, who owns two other runner-up finishes this year, needed an eagle at the last to force a playoff, but only birdied it to share second.
Peoples was on fire through most of his round. He birdied three of the first five holes to get to minus-13. Starting at the eighth, the 45-year-old ran off three straight birdies around the turn.
The two-time winner on the PGA Tour then eagled the 11th to jump to 18 under. Peoples joined the leaders at minus-20 with birdies on 13 and 14. He moved into the lead outright with a birdie on 16th. However, he stumbled to a bogey on 17 and parred the last to end one back.
Bowditch picked up three birdies in the first eight holes. He ran in three birdie putts in a four-hole stretch from the 11th to climb to 18 under. The Australian, who won at the Jacob's Creek Open earlier this year, eagled the par-5 closing hole to get to minus-20.
Veazey parred his first 11 holes to remain at minus-15. He finally got into red figures for his round with a birdie on the 12th. The 40-year-old birdied the next three holes as well to move to minus-19. The Panama Championship winner ended one shot shy of Tambellini with a birdie at the last.
Johnson Wagner, who shared the lead after round one, and Pat Bates each closed with 65s. They ended at 18-under-par 270 alongside Scott Weatherly (68), Gabriel Hjertstedt (69), Jerry Smith (66) and Kevin Johnson (68).