A Wales Under-21 international, Harris joins after a successful loan spell with last season’s League One champions Birmingham City, playing 36 games and scoring four goals.
The 20-year-old has also played 21 times on loan for Exeter City in League One, scoring a further four goals.
With seven first-team appearances for Fulham, four coming in the Premier League, the attacking midfielder is one of the London club’s most exciting young prospects.