Mersey Island is the smallest, though not the least populated, of the 32 main islands of the Southern Arc Islands. It is covered almost completely by the town Mersey. The highest point of the island is Mersey Hill (217 m), on which Fort Mersey (surrounded by the Mersey Hill Domain) stands.
The island belongs to the Mersey Island District. Its total area is approximately 25 km2.
- Mersey: 13'056
- Other: 0 (the Mersey urban area, combined with natural areas, covers almost all of the island)
- Total: 13'056