Decker Mountain is a 2,421-metre (7,943-foot) flat-topped peak located in the Garibaldi Ranges of the Coast Mountains, in Garibaldi Provincial Park of southwestern British Columbia, Canada. It is part of the Spearhead Range, which is a subset of the Garibaldi Ranges. It is situated 9 km (6 mi) southeast of Whistler, and 3.9 km (2 mi) west-northwest of Tremor Mountain, which is the highest point in the Spearhead Range. Precipitation runoff from the south side of the peak drains into Fitzsimmons Creek which is a tributary of the Cheakamus River, and meltwater from the Decker Glacier on the northeastern slope drains to Wedge Creek.