Slattery had been chasing back-to-back European Tour victories after winning last week's Russian Open after a seven-under-par 63 in the third round left him sharing the overnight lead with Spain's Rafa Cabrera-Bello.
Instead it was Pieters who took a back-to-back Tour win, following up his Czech masters victory last month, when Slattery missed a four foot putt for par.
"I'm a little bit surprised because I was getting ready for the play-off," said the Belgian after a final round of 65 to finish 19-under-par-261 and one clear of Slattery and Spain's Eduardo de la Riva.
"Lee played really well but this victory is even cooler than the first one and now I want number three," added Pieters, who had been two strokes down overnight but fired four birdies on the front nine on Sunday.
"I felt like I played better in the Czech Republic but my short game was great this week and saved me many times."
(Reporting by Alan Baldwin, editing by Pritha Sarkar)
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