The 18-year-old joins Albion on a deal that runs until June 2018 from Southern Premier League side Dorchester Town, for whom he signed last summer, and he will initially link up with Simon Rusk's under-21 squad.
Academy manager John Morling said, "Jonah is somebody we became aware of earlier this season, and we are delighted to welcome him here to the club.
"He is a very exciting young talent. He is a strong, quick and powerful centre-forward, who has impressed in a short space of time at Dorchester.
"Our recruitment team has been monitoring him during that period and he comes here very highly recommended. I am confident he will prove to be a great addition to Simon Rusk's under-21 squad."
Beaminster-born Ayunga made a total of 28 appearances for the Magpies this season, finding the net seven times. He previously played for Bridport's youth team and featured regularly for the Dorset representative side.
Albion's under-21 match against Wolves, at Crawley Town tonight, comes too soon for Ayunga, but he could be in line for his under-21 debut at Arsenal on Friday lunchtime.