The Spaniard, who claimed his 21st victory on the European Tour, shot a two-over-par 73 in the final round for a total of 284, forcing a playoff against the 22-year-old Pieters (75) and Australian Green (72).
The 50-year-old Jimenez held his nerve as the trio went back down the 18th hole and with Pieters and Green spraying their tee shots, world number 34 Jimenez tapped in for par to seal his first Spanish Open victory in 27 attempts.
"I've played so many times in the Spanish Open, it's great to finally win it, it's amazing to win the national Open of my country," home favourite Jimenez told Sky Sports.
"Every title is special, and you don't like to say one is better than the other but to win the Spanish Open is very, very important to me."
- Sports & Recreation