The 19-year-old struck after 19 minutes, slamming a rising shot past the outstretched hands of goalkeeper Chiamaka Nnadozie and into the corner of the net.
It was the first goal of the tournament for Nyadjou, and it earned Cameroon a semi-final showdown on Wednesday against hosts Morocco in Rabat, and a ticket to the 2027 World Cup in Brazil.
Record 10-time WAFCON champions Nigeria pressed for an equaliser in the closing stages only to be constantly defied by acrobatic saves from Michaely Bihina.
The closest the Super Falcons came to equalising was in the third minute of added time when Esther Okoronkwo won a race for possession with Bihina, but her shot trickled wide of the far post.
Just before the miss, Nigeria introduced six-time African Women’s Footballer of the Year Asisat Oshoala, who was not sufficiently fit to start.
But the Saudi-based striker was unable to make an impact and Cameroon comfortably survived the last few minutes for only their second victory over Nigeria in 14 WAFCON meetings.
As the Namibian referee signalled full-time, the Cameroon substitutes and coaches raced on to the pitch and hugged the Indomitable Lionesses.
Later in Rabat, debutants and giant killers Malawi take on three-time runners-up Ghana and the winners will meet Algeria in the other semi-final.
