The 31-year-old German became the oldest ATP tour winner this year after collecting his 12th career title in a tournament seen as a major warm-up for Wimbledon. Haas has now won titles on all surfaces.
"It's a great accomplishment for me,'' Haas said. "It's such a great feeling to hold a trophy again, it makes all the setbacks go away.''
It was his first title since winning in Memphis in February 2007 and his first final since then.
"It means a lot to me to beat a player as classy as you in the final,'' Haas said during the victory ceremony as he turned to Djokovic.
The sellout crowd of 11,500 gave Haas a standing ovation.
Haas was ranked No. 2 in 2002, but injuries have hampered his career in recent years and he had his third shoulder surgery in November 2007. He entered this week's tournament ranked No. 41 and will jump up about five spots.
"It's incredible, I can hardly believe it myself when I think of the highs and lows I've been through in the last year and a half,'' Haas said. "It's been a dream week.''
The German had his best showing at the French Open in seven years when he reached the fourth round, losing to eventual champion Roger Federer in five sets after winning the first two.
"Right now, I am not thinking of Wimbledon at all, I am just enjoying the moment because I don't know if I'll have many more moments like this,'' Haas said. "But I surely will have more confidence going there, and I am leaving here with a big smile.''
Djokovic was in his second career final on grass, after Queens last year.
A former Australian Open champion who is now ranked No. 4 in the world, Djokovic lost his fourth final of the year. He has two titles in 2009.
"I did not play well at all,'' Djokovic said. "I was lucky to win the second set.''
Haas never faced a break point in the first two sets but he had two double faults in the tiebreaker to allow Djokovic to take a 5-2 lead. Djokovic lost the next two points, however, including one when he chose to make a rare advance to the net behind a second serve. Haas had the chance to pull even, but instead put a backhand wide. Facing a set point, Haas produced his third double fault.
"I would have lost the set without those double faults,'' Djokovic said.
After a long first game in the third, Djokovic double-faulted twice in dropping his serve in the second game. Haas faced his first break points of the match in the next, but he saved all four of them to go up 3-0.
Haas nearly went up 4-0, but Djokovic saved three break points to win his first game in the third.
The German used a big forehand passing-shot winner to break again for 5-1 and served out the match at love, hitting a volley winner after two hours and 12 minutes and beating Djokovic for the first time in three meetings.
"I didn't put any pressure on his serve,'' Djokovic said. "I served very badly, I did not return well, I gave him many opportunities. But it was a good week and getting to the final is a good achievement.''
"I had enough matches on grass before Wimbledon but I would not call myself one of the favorites though,'' said Djokovic, who has looked uncomfortable on the grass surface and had to save five match points against Florent Serra in the second round.
Djokovic said he was still recovering mentally from a tough loss to top-ranked Rafael Nadal in the semifinal of the Madrid Masters, when he wasted five match points.
"That loss exhausted me mentally, I am still trying to recover,'' said Djokovic, who then lost in the third round of the French Open.
