Phillips, 28, has signed a three-year deal with the Baggies and joins for an undisclosed fee.
West Brom sporting director Andrew Nestor stated, "Nat will bolster an already strong back line and brings with him leadership qualities, which really stood out for us.
"We believe he is entering his peak years as a centre-back, and hope to see him develop even further with us here as a player and leader within our group."
Putting down new roots
Phillips said: "I think everyone at the club and involved would like to see it competing for promotion and pushing for Premier League football - and that's the same as me.
"I've had a number of loans over the past few years and it'll be really nice for me and my family to go somewhere and know we're staying there to develop a relationship with the club and the fans."