The extension, until 5 p.m. ET on April 15, comes a day after Osisko announced a friendly deal involving Yamana Gold (TSX:YRI) and two of Canada's largest pension funds.
The bid was set to expire Friday.
Goldcorp said it will take the extra time to examine the rival offer and conduct its own due-diligence investigations of Osisko and its main asset, the Canada Malartic gold mine in Quebec.
The friendly deal which values Osisko at $3.4 billion would see Yamana acquire a 50 per cent stake in the gold miner's operations and exploration assets, but Osisko would remain an independent company.
Under the offer, Osisko shareholders would receive $2.194 in cash, 0.2119 of a Yamana share and a new share of Osisko.
The companies valued the offer at $7.60 per share based on a value of $3.35 for the new Osisko share.
Goldcorp, which is one of Canada's largest gold miners, has offered Osisko shareholders 0.146 of a Goldcorp share plus $2.26 in cash for each Osisko share, a deal worth about $6.21 per share as of Tuesday.