Short answer: it really depends on the type of bench you buy. Many affordable park benches from big-box stores and online sellers arrive in flat boxes, so you will need basic tools. Commercial-grade benches from specialty suppliers often arrive fully assembled or close to it.
If you are picturing a traditional heavy park bench with cast iron ends and metal or wooden slats, those are commonly shipped pre-assembled when purchased from a commercial park furniture company. They are bulky and heavy, so sending them in pieces does not make much sense. Even then, you may need a wrench or drill if you plan to anchor the bench to concrete or pavers, but the bench itself usually comes in one piece.
By contrast, most residential-style benches sold at hardware stores or online—especially teak, eucalyptus, or metal slat benches—come partially disassembled. A typical box includes side frames, seat slats, a backrest, armrests, and a hardware kit. The good news is the hardware is normally included, and the only tool you may need is an Allen key. I would still keep a screwdriver, an adjustable wrench, and a rubber mallet close by, because some bolts need extra tightening once the bench is sitting on uneven ground.
You will also see different wording in product listings. “Some assembly required” usually means 30 to 60 minutes of work, especially if you are doing it alone. “Fully assembled” means the main body is ready to use, but you might still attach the legs or arms. “No assembly required” is the phrase to look for if you want zero tool time.
My practical advice: if you want a bench you can unbox and sit on immediately, search specifically for “fully assembled park bench” or call the seller and ask whether it ships in one piece. If you are okay with light assembly, a flat-pack bench can save money and is easy to handle with basic tools. When assembling, lay the parts on cardboard or a blanket to avoid scratches, start all bolts loosely, align everything, and tighten evenly. That prevents a wobbly bench later.
So yes, in most cases you will need at least a simple set of tools—but unless you order a fully assembled commercial bench, it is rarely a difficult job.