The 23-year-old saw off competition from Mats Hummels and Robert Lewandowski to secure the accolade, with the trainer's part in Dortmund's title win also recognized.
The 23-year-old led the Foals' surprising charge to a fourth-placed finish with 18 goals and a further 11 assists, earning a move to Borussia Dortmund in the process.
Reus won a total of 217 votes from the German Association of Sports Journalists, beating his now Schwarzgelben teammates Mats Hummels, who received 108 votes, and Robert Lewandowski, who got the nod from 95 writers.
Despite Gladbach's excellent campaign, Lucien Favre was unable to secure the award as the best coach, finishing second with 138 votes, with Dortmund boss Jurgen Klopp scooping the accolade with 496.
The Ruhr giant put together a 28-match unbeaten run to retain their Bundesliga title ahead of Bayern Munich, and completed a domestic double by beating the Bavarians 5-2 in the DFB-Pokal final.
Borussia Dortmund will have the chance to get its 2012-13 campaign off to a winning start when they face Jupp Heynckes' side in the DFL-Supercup on Sunday.
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