As a free agent following his decision to leave Norwich City at the end of last season, the 36-year-old trained with his new team-mates during the global break and soon convinced Albion's manager Darren Moore that he would be a valuable addition to his numbers.
Hoolahan ended a 10-year spell with Norwich at the end of last season and has been without a club since, but a trial during the worldwide break convinced Baggies boss Darren Moore to sign the Irishman.
"We had a look at him over three or four days". He is a great pro and I think everyone knows the quality he possesses. I'm delighted to get him here.
Baggies head coach Darren Moore told the club website Hoolahan's "quality was there for all to see".
After being relegated from the Premier League last season, West Brom began their new campaign with a defeat and a draw, but have recovered to win three of their last four to move within four points of leaders Leeds United. I know what it's like to play into my mid-30s and the areas of sports science and medicine have developed even further since my day. "He will get even better with game time but in terms of his general fitness he has satisfied all our demands".
Fellow Norwich legend Russell Martin has also been training with the Baggies but reports - which were accurate on Hoolahan's imminent signing - have suggested the Scotland worldwide defender will not be joining Moore's squad.