The 21-year-old is now committed at Town until the summer of 2026.
Pontefract-born Harratt, who spent a loan spell at Bradford City last term and has also spent a temporary stint earlier in his career at Harrogate Town, impressed towards the tail end of last season under Neil Warnock.
He scored his first competitive home goal in the recent EFL Cup tie against Middlesbrough, having netted in last season's fine victory at Watford in the club's 'Great Escape' from relegation.
Town chief Neil Warnock commented: "I’m absolutely delighted for Kian.
"Since I’ve been back at the club, he’s shown an attitude to his work that I’ve not seen in a player in my managerial career to date.
"He’s been like a breath of fresh air, and he deserves everything that he gets.
"This is only the first rung of the ladder to climb, but I’m sure he’ll have a successful career."
Town Sporting Director Mark Cartwright added: "We’re really pleased to have Kian extend his contract with the club, he’s worked incredibly hard to earn this and it’s a deserved reward.
"Similarly to Kyle Hudlin, his next challenge is to break into the first team on a more consistent basis as he’s shown the hunger and ability in front of goal to score at this level in the past few months.
"It is always great to see someone who’s been in your academy continue to progress at your club, and if he continues to work as hard as he is currently, anything is possible."