Chadli agreed a four-year contract at the Premier League club, but the Belgium international's fee is set to be eclipsed shortly if Albion complete their proposed £15 million deal for Malaga's Ignacio Camacho.
Chadli, who spent three years struggling to hold down a first team berth at Tottenham, becomes only Albion manager Tony Pulis's third signing since the end of last season.
"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," Chadli told Albion's website.
"My ambitions? I just want to help the team win as many games as possible."
Pulis added: "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, 27, arrives after making 119 appearances and scoring 25 goals for Tottenham after joining from Twente for £7million.
He played 40 times for Spurs last season but has not featured for Mauricio Pochettino's men this term.
Albion have suffered transfer difficulties in recent weeks, with Pulis voicing his concerns over a lack of players several times.
They pulled out of a £15 million deal for West Ham's Diafra Sakho in July over fears with his fitness, had several bids rejected by Leicester for Jeff Schlupp and have been chasing Southampton's Jay Rodriguez on loan.
Chadli's arrival paves the way for Callum McManaman to leave, with Newcastle linked with the winger after Pulis confirmed he expects the former Wigan man to depart before Wednesday's transfer deadline.
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