The 19-year-old attacking midfielder joined the League One club from Tottenham Hotspur's foundation college programme last May.
His eye-catching strike against Bury in November was voted the club's goal of the season in a debut campaign which included 12 starts in all competitions.
Irishman Osadebe's new deal at Priestfield Stadium includes the option of a further one-year extension.
Emmanuel Osadebe has signed a new one-year deal with Gillingham.