Portugal midfielder Fernandes, who joined United in 2020 and has scored 79 goals and provided 67 assists in 234 appearances for the club, had signed a previous contract in 2022 which ran until 2026.
"Everybody knows the passion I have for Manchester United," he said in a statement.
"I understand the responsibility and significance of wearing this shirt, and the levels of dedication and desire required to represent this incredible club.
"I wouldn’t have signed this contract if I didn’t believe that my best moments in a United shirt are still to come."
FA Cup winners United begin their Premier League campaign at home on Friday against Fulham.