Darren Fletcher completed a deadline day move from Manchester United after joining the Baggies on a free transfer.
The United vice-captain joins the Throstles on a two-and-a-half-year deal, with a year in the club’s favour, after completing a medical at The Hawthorns.
Albion agreed a deal with the Scotland international after his move to West Ham collapsed.
But the Baggies suffered disappointment on deadline day when a deal for Carlton Cole was pulled by the Hammers and Norwich defender Martin Olsson failed to agree terms.
Fletcher, who has only ever worn the red of Manchester, said his move to The Hawthorns felt ‘great’ and paid tribute to Baggies boss Tony Pulis in convincing him on the switch.
"I've spent my career at Manchester United and it's all I've ever known,” said the 31-year-old.
"But I felt it was the right time to move and I'm not sentimental about doing that - I have great memories obviously, but I'm now a West Bromwich Albion player and I cannot wait to get going.
"Everyone involved in making the deal happen has been fantastic. And a big reason I'm here is because of the head coach.
"He (Pulis) has impressed me from the first minute. When someone shows how much they want you it's a great feeling.
"I'm a midfielder who is capable of playing wherever needed - whether it's box-to-box, or sitting in deep, getting on the ball, tackling, breaking up play, etc,
"I know a few of the lads too. Mozza, who I know well from Scotland duty, has been pestering me for the last week, texting me constantly. He was giving me glowing references about how much I would love it here."
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