Watson had a loan spell at Queens Park Rangers earlier in the season.
“Having lost Gonzalo Jara for the remainder of the season to injury, we have been looking to strengthen the central midfield area and we are very pleased that Ben has joined us,” Baggies boss Roberto Di Matteo told the club’s official website.
“Ben is very good technically and is a competitive player who knows the Championship very well.
“He has played many times at this level and knows what is required, which should help him to hit the ground running.”
