The 23-year old has signed a two-and-a-half-year contract, with a further one year's option also in the club's favour. The former Dutch Under-21 international has been a regular for the reserves this term, scoring an impressive six goals in return.
Brought initially to the club as a forward MacDonald's most recent appearances for the Baggies have come on the left wing with the pacy forward's natural ability impressing Hawthorns boss Tony Mowbray.
"Talent is hard to come by and when you've got someone with the ability Sherjill has to beat an individual in a one-on-one situation, we shouldn't be discarding him," Mowbray said.
"Sherjill has got talent and potential. We need to work with that, develop it and hopefully make him a big asset for our football club."