Despite the global significance of the Leach’s Storm-petrel Hydrobates leucorhous colony on Baccalieu Island, Newfoundland and Labrador, Canada, the estimate of 3.36 million breeding pairs reported for 1984 by Sklepkovych and Montevecchi stands as the single published population estimate for the world’s largest colony. This study increases knowledge of this population by analysing data from additional independent surveys conducted in 1984 and 1985, and by updating the population status with a survey conducted in 2013. Population estimates were derived by extrapolating occupied burrow densities to the estimated occupied area of four main habitat types (heath, forest, grass and fern), which in turn were based on proportions of habitats observed in plots (1984 and 1985) or by using a Geographic Information System approach (2013). Based on these surveys, the Leach’s Storm-petrel breeding population size on Baccalieu Island was estimated at 5.12 ± 0.73 (SE) and 4.60 ± 0.42 (SE) million pairs in 1984 and 1985 respectively, representing estimates 37–51% greater than the original 1984 survey. While discrepancies among these estimates were largely driven by the way occupied areas were estimated, our study confirms that Baccalieu Island hosts the largest Leach’s Storm-petrel colony in the world. Results from the 2013 survey estimate the current breeding Leach’s Storm-petrel population at 1.95 ± 0.14 (SE) million pairs, representing a 42% decline over 29 years (-1.4% per year), relative to the original published estimate of 3.36 ± 0.12 (SE) million pairs. The most prominent change has occurred in the density of storm-petrel burrows found in forest habitat which dropped by 70% despite forest remaining the second most abundant habitat available to nesting storm-petrels on Baccalieu Island. The cause of this decline remains unknown and is likely multi-faceted. Future research focusing on demographic studies is required to understand what is driving the population decline of this internationally important colony.