The Belgian has signed a four-year deal with the Baggies after passing a medical today.
West Brom manager Tony Pulis said: "He’s a top, top player and I’m delighted we’ve got him.
"I said last week that these signings all had to be about players who would improve our squad – and Nacer does precisely that".
Chadli himself added: "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.
"My ambitions? I just want to help the team win as many games as possible".
Chadli played 119 games for Spurs in three years and scored 25 goals for Pochettino's side.