Dekker, 21, has signed a one-year deal and could feature in Melbourne City's FFA Cup clash against Edgeworth Eagles on Tuesday evening.
"The club's coaching staff have been monitoring Wade's progress closely over the last 12 months and we believe he has matured into a very promising attacking player," Melbourne City coach John van't Schip said.
"A senior contract is great reward for his application, professional attitude and consistent performances.
"Last year we elevated Paulo Retre from our youth team and he re-signed again for this season, so this is a great opportunity for Wade.
"Promoting players from within our system is very important to us and with teams in the NYL and NPL, hopefully it is only the beginning."
