Osprey Ultralight Pack Liner

The Osprey Ultralight Pack Liner is a waterproof bag that sits inside your backpack as the last line of defense against rain — unlike a rain cover, it protects the contents even if water gets in through the pack's seams or zippers. It's made from recycled 40D bluesign-certified nylon and closes with a roll-top system that becomes effectively watertight once rolled down a few times and buckled. The seams are sealed rather than just sewn and taped, reducing weak points where water could seep in during sustained rain or a stream crossing. The size reviewed here holds roughly 58 litres (Osprey lists it for the 50–70 L range) and weighs 116 g — a negligible weight penalty given the protection it offers. It works well paired with a pack that has no waterproofing of its own, or as backup to a rain cover on multi-day trips where a wet sleeping bag or soaked electronics would be a real problem.