Nabi is a promising talent who has been a regular for their under-21 side, but had failed to make the jump into the first team.
Earlier this season he made a surprise loan move to Indian side the Delhi Dynamos, and has recently returned.
This campaign needed to be a big one for him, and if Tony Pulis was not intending on giving him a first team chance then a permanent exit is for the best.
The deal is a little risky for the Baggies, who have had Nabi coming through their ranks for several seasons now, and letting him go without giving him a chance may leave them regretting it, with former youngster Kemar Roofe excelling for Oxford United this season, scoring 23 goals in all competitions.
Nabi, 21, is eligible to play international football for both England and Pakistan, and has represented the Three Lions at under-17 level.
In 74 under-21 games for the Baggies he scored 26 goals and provided 13 assists.
He will not be eligible to play against West Brom when they take on Peterborough in the FA Cup later this month.
Here is a look at the split reaction from West Brom fans to his departure...
Another player who was meant to be future prospect sold hmm yeah!!! Good luck to Nabi #wba— Cj (@cjbaggie) January 21, 2016
Very surprised with the Adil Nabi news after how highly he has been spoken of!— Andy Pitt (@andyjpitt) January 21, 2016
Like Kemar Roofe and Bradley Garmston before him, Adil Nabi is a player I believe we are letting go too soon. #WBA— Sunny Randhawa (@SunnyRandhawa) January 21, 2016
Selling Nabi will come back to bite us— caswell (@WestBrom1996) January 21, 2016
People going mad because Nabi has left. He's never played a senior professional match in England. Light years between league 1/2 to PL #wba— Andy Smart (@AndySmart88) January 21, 2016
Nabi was obviously not good enough or else he wouldn't now be playing for Peterborough before anyone starts#wba— Lewis (@lewis_harbon) January 21, 2016
Why on earth have we let Nabi go? It's just gonna be another Roofe situation... #wba— Mason (@Mason7WBA) January 21, 2016
Jesus wept just seen about Nabi there gone without even making a first team appearance. This Academy JP loves isn't producing is it?— Gavin Poulton (@gavin_poulton) January 21, 2016