Great question — and a smart one if you live somewhere with high humidity or frequent rain. The short answer is: it depends on the material and the manufacturer.
Most reputable brands that sell high-quality outdoor benches do offer warranties that cover warping, but only if the bench is made from materials specifically designed to resist moisture. For example, benches made from kiln-dried teak, powder-coated aluminum, or high-density polyethylene (HDPE) are far less likely to warp in humid climates. These materials expand and contract minimally with moisture, so manufacturers often back them with a 5 to 15-year warranty against warping, cracking, or rotting.
However, if you're looking at a bench made from untreated wood or inexpensive pine, don't expect a warping guarantee. Those materials are prone to absorbing moisture, which leads to twisting and splitting — and most manufacturers will exclude "natural weathering" from the warranty.
To be safe, always read the fine print. Some warranties specifically say "limited lifetime" but exclude problems caused by humidity. Others offer a "no-warp" guarantee but only for the frame, not the seat slats.
So before you buy, ask the seller directly: "Is this bench guaranteed against warping in high-humidity environments?" And if they say yes, ask if that's in writing. That way, you'll relax — and your bench won't warp.