The 38-year-old overcame a two-shot deficit to compatriot Bradley Dredge on the final day, who finished second on 12-under.
South African Merrick Bremner and Denmark's Soren Kjeldsen finished tied for third, a shot further back.
Scot Stephen Gallacher, another European Ryder Cup hopeful, finished tied for seventh, seven shots behind Donaldson.
"It's been an amazing week," Donaldson told Sky Sports. "I didn't decide to come here until late. I needed to play well this week or next week or both to guarantee a place in the Ryder Cup team. "There's a lot on the line and I knew I had to play well and this week I played really well and it all came together."It's so difficult to get in the team. I've played as well as I can play for 18 months. A couple of slow events and you just rocket down."
With his short game in particularly fine fettle Donaldson started his final round in inspired form, taking the lead with three birdies from the first three holes.
He bogeyed the fourth to surrender the advantage to Kjeldsen who birdied the fifth hole.
The lead continued to change hands until Donaldson made the most of two Kjeldsen bogeys to open up a two-shot lead with a long birdie at the tenth.
From that position he rarely looked in trouble and could even afford to bogey the 17th hole on his way to the title.
(Reporting by Tom Hayward; editing by Josh Reich)
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