The solution to this question depends on the type of lettuce. If you have crunchy lettuce like romaine, it will last longer washed because moisture helps keep it hydrated and crisp.  If you have a more delicate leafy green like butterhead or red leaf, then it is best to store it unwashed. Washing these types of lettuce can hasten spoilage.  To ensure maximum longevity, store unwashed lettuce in a sealed, airtight container in the refrigerator.  Alternatively, you can wash and dry the lettuce, wrap it loosely in damp paper towels or cloths and place it in an open bag in the fridge for up to 5 days.  To further extend its shelf life, try adding a few slices of onion to the container. This will help keep the lettuce fresher for longer.