The 27 year-old Belgian moves to The Hawthorns on a four-year deal and usurps the £12m amount spent on striker Salomon Rondon from Zenit St. Petersburg last August.
Chadli becomes just the second signing of the window following the arrival of Matt Phillips from QPR earlier this summer.
Belgian out of favour in North London
Chadli moved to Whit Hart Lane in July 2013 from FC Twente for a fee of £7m.
In 119 appearances for The Lily Whites, the Belgian scored 25 goals for Maurico Pochettino's side.
The winger has been predominantly a feature from the bench during his three years in North London. After further investment in Victor Wanyama and Vincent Janssen in the build-up to the new campaign also, Chadli is yet to feature this season.
Chadli 'very pleased to be here'
Speaking to the club's official website, The Baggies latest import stated, “I feel very good to be here. It came very quickly but I spoke to the club and they have a good team, a good manager and I am very pleased to be here."
On arrival Chadli spoke of his ambitions and desires of his timein the West Midlands. “My ambitions? I just want to help the team win as many games as possible.” In regard to his role for West Brom, the Belgian feels, “I think my best position is out on the left", but that "I can play anywhere in midfield if asked."
“This is a great challenge" Chadli states, adding he wants "to show my quality to the fans and to the club."
With just three days remaining until the transfer window slams shut, Tony Pulis has spoken of bringing in up to five new faces into the club. The Baggies boss is set for busy few days to come.