Littler won last year's title on debut and was good value to retain his crown as he averaged just shy of 103.
Victory in Wieze ensures the 18-year-old adds the title to the World Championship and UK Open he has won in 2025.
"I'm very happy to retain the title in front of these great fans," Littler said after the final.
"I said after the World Championship, if I don't win anything throughout the whole year, I'm still a world champion.
"I'm happy to win the UK Open and come back here and go back-to-back; now it's straight onto the Players Championship events and then back to the Premier League."
Littler had beaten Luke Woodhouse, Dave Chisnall and James Wade earlier in the day to reach the final, where he quickly opened up a 3-1 lead.
But 'The Nuke' was challenged by De Decker, who had been in good form on the day after getting past Martin Schindler and Gerwyn Price and fighting back to down Ross Smith.
The Belgian managed to break Littler three times across the match, although that resistance wouldn't phase the teenager.
He held firm during the raucous atmosphere to see out victory before being showered with a bottle of water by De Decker in celebration.