Slowed by injuries and illness for two seasons, Williams showed she still had the winning touch and played steady tennis while Niculescu proved error-prone as she sought her first career title.
At 32, Williams won her 44th singles title in a career that includes seven Grand Slams.
The 70th-ranked Niculescu lost in the Luxembourg Open final for the second straight year.
Williams, now ranked No. 41, had not won a tournament since early 2010, when she took the titles in Dubai and Acapulco.
Throughout the 1-hour, 22-minute match, Williams rarely looked in trouble, taking the early initiative and never letting go.
For a player who relies strongly on her serve, she was successful on only 63 percent of her first serves and didn't manage a single ace on the indoor hard-courts at CK Sportcenter Kockelsheuer. She was much more solid in her backcourt game as Niculescu looked hesitant from the start and failed to get any momentum going.
The victory was a welcome turnaround for Williams, who has been struggling for several years from an autoimmune disease, which often left her fatigued and with painful joints.
She only started her season in March following a seven-month layoff because of the illness. And even then, it often was a slog through the summer in singles play, as she struggled to capture the style and power of old.
In doubles, she did shine when she won a 13th Grand Slam doubles title at Wimbledon with sister Serena and then made it a repeat performance at the London Olympics a month later.
That form did not rub off in singles play though. She hasn't made it past the fourth round of a Grand Slam since the 2010 U.S. Open and among the fans speculation over potential retirement started making the rounds.
Williams would have none of it. The first signs of improvement came when she reached the semifinals in Cincinnati in the run-up to the U.S. Open despite some back problems.
She was ousted in the second round at the U.S. Open, but in her first tournament since, she finally ended her title drought.