The word knapsack can refer to:

• a backpack
• Knapsack, Germany, a locality of Hürth, Rhine-Erft district, North Rhine-Westphalia
• the knapsack problem, a math problem

• the subset sum problem, a special case of the above
• Naccache-Stern knapsack cryptosystem, a cryptosystem based on the knapsack problem

• Knapsack (band), an American band
• "Knapsack!", an episode of Aqua Teen Hunger Force