The deal will keep the highly-rated striker at The New Den until 2005 and will disappoint a number of clubs, who were aiming to prise him away from the club.
Middlesbrough were understood to be keen on signing the 23-year-old ace, who grabbed 28 goals last season, but Harris admitted he was delighted to pledge his future to the London outfit.
"This should make it clear to anyone who may have had any doubts I'm totally committed to playing for Millwall for the forseeable future," Harris told the club's official website.
"I've been saying it for some time I don't want to go anywhere else and this should be the proof any doubters need.".
"The chairman said at the start of the season that if we won promotion there would be new deals up for grabs and he's been as good as his word.
"I know a number of the other lads will also be signing new contracts, and it's a great time for so many of us to be committing ourselves to the club for a longer term.
"We see the Second Division championship as just the start of something bigger, and we're realistically looking for a top six finish in Division One next season."
