The Magpies have signed the 16-year-old winger from Independiente del Valle, although the youngster will not officially join the club until after his 18th birthday in the summer of 2027.
Martinez will remain with Independiente del Valle for the next 12 months, having been part of the club’s youth team that won the Ecuadorian Under-17s title last December.
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The teenager is currently on international duty with Ecuador’s Under-17s at the South American Under-17s Championships, with his goal in Sunday night’s win over Chile helping his side reach the semi-finals.
Newcastle’s sporting director, Ross Wilson, said: “We’re delighted to beat off serious competition from leading clubs across Europe to bring Johan to Newcastle United.
“Identifying and recruiting promising young players is a key part of how we build for the future, and our scouting and recruitment teams have worked diligently to secure Johan’s signature.
“Independiente del Valle have a strong reputation for developing young players and Johan has undoubtedly benefited from that environment. He will continue his progress there while we maintain a close connection with his development, and we look forward to welcoming him to Newcastle next year.”