Fifteen is a composite number with proper divisors 1, 3 and 5.

Also:

• A pentadecagon has 15 sides
• 15 is a triangular number
• 15 is a hexagonal number
• 15 is a lucky number
• A pentagonal prism and a pentagrammic prism have 15 edges
• 15 is the magic constant of the unique order-3 normal magic square.