The 24-year-old also won the audience prize, voted for by listeners and TV viewers.
She was among five finalists performing before a jury at St David's Hall in Cardiff on Sunday.
Afterwards, the singer said: "I am so happy, I feel I'm in heaven right now. It is everything to me."
Meeta Raval from England, Olysya Petrova from Russia, Hye Jung Lee from South Korea and Andrei Bondarenko from Ukraine also competed fro the £15,000 main prize.
Baritone Bondarenko was the winner of the international competition's second award, the Song prize, on Friday in which Nafornita also sang.
Dame Joan Sutherland prize
Nafornita performed a programme of composers Donizetti, Dvorak and Gounod to win over the panel of judges.
The judges for the competition included Dame Kiri Te Kanawa, Russian-born conductor Alexander Polianichko, Lorenzo Mariani the artistic director at Palermo Opera House in Italy, mezzo-soprano Marilyn Horne, tenor Dennis O'Neill and baritone Hakan Hagegard.
As well as the main prize, the audience prize of £2,000, was also awarded to Ms Nafornita.
It is chosen by a public telephone vote and has been renamed the Dame Joan Sutherland Audience Prize in honour of the soprano's long association with the competition.
Dame Joan was the patron of the competition until her death last year.
Six hundred opera and concert singers auditioned for a place in the competition.
Twenty singers from around the world took part in the week-long biennial contest last week.
The event was established in 1983 by BBC Cymru Wales. Past winners include Karita Mattila, Dmitri Hvorostovsky and Ekaterina Scherbachenko.