The 16-year-old completed her solo round-the-world journey when she sailed into harbor on the Caribbean island of St. Martin, which is shared by Netherlands and France, at 3:00 pm (1900 GMT).
Dekker, who left the island nearly a year ago to the day -- January 20, 2011 -- beat the previous record by some eight months.
The previous record holder was Australian Jessica Watson, who achieved it in May 2010, three days before she turned 17.
But unlike Watson, who circumnavigated non-stop, Dekker sailed from port-to-port and was never at sea for more than three weeks.
Dekker's achievement will not be entered in the Guinness Book of World Records however, as it has refused to recognize records by minors which are considered "unsuitable".
Neither will the World Sailing Speed Record Council -- the official body that validates such records -- acknowledge the feat.
"All that matters is speed, we don't do any personal records, age doesn't matter," John Reed, the council's secretary, told AFP.
The young sailor told a press conference, held just minutes after her return to dry land: "I can't really take in what I've done."
"Sometimes I've asked myself what I was doing," she added after an emotional reunion with her parents and sister.
"The sailing was always nice... I often saw dolphins. I enjoyed it a lot and I'm very happy," Dekker said.
"I'll spend the next few days cleaning the boat," the 11.5 metre-long (about 38 feet) ketch, called Guppy.
After that it's back to school, she added.
Her parents were among a 400-strong crowd of well-wishers -- on shore and in small boats -- who welcomed the teen, dressed in a black tee-shirt and beige shorts.
Fight to get to starting line
Just to get to the starting line, Dekker had to fight her way through the Dutch courts, who at first blocked plans for her to cast off a year earlier -- when she was just 14.
The court ordered her placed in the care of welfare officers on the grounds that she was too young to guarantee her safety at sea.
She ran away to St. Martin, and police had to escort her back home.
She finally won her court battle with Dutch child welfare authorities in July 2010 -- after 10 months -- and set sail, originally from Gibraltar on August 21, 2010 in her yacht Guppy.
But a change of her planned course led her to make the starting point from her trip St. Martin instead.
Born on a boat in New Zealand of a seafaring family, Dekker also lived on a boat in the Netherlands with her father Dick and dog Spot before setting out on her voyage.
