Williams dispatched Niculescu 6-2, 6-3, snapping the longest victory drought in the 32-year-old former world No. 1's career.
Her previous victory came at the 2010 Acapulco event.
"Coming to the tournament this week I was just trying to play well," Williams said. "I didn't know if I could win this event, so to stand here as the winner is a wonderful way for me to end the year, and it really makes me look forward to next year as well."
Niculescu was appearing in just her second career final after falling to Victoria Azarenka at the 2011 Luxembourg event.