The company said it would buy for cancellation up to 25.51 million Class A shares. The public float comprised 255.13 million shares as at December 14, and 391.25 million total issued and outstanding on the same date.
The programme will expire on December 23, 2019. All purchases at prevailing market prices will be made via the TSX, alternative Canadian trading systems and/or the NYSE.
Alamos said the buyback would be value accretive for shareholders.
Alamos shares rose as much as 7.2% or C32c Thursday before closing 29c higher. The company is still trading 40% below its year-earlier level, with a current market value of $1.85 billion.