Short answer: yes, you can lock a street bench to the ground with a chain, but it is rarely the best system. A chain can work as a temporary fix or a low-security deterrent, but for anything permanent or placed in a public area, I would use a direct ground anchor instead.
The problem with a chain is that it creates a trip hazard, usually looks messy, and can often be cut quickly with bolt cutters or an angle grinder. You also still need a strong anchor point. Wrapping a chain around a tree, signpost, or fence may damage property or violate local rules. If you do use a chain, it should be a hardened square-link chain at least 10–12 mm thick, paired with a shielded padlock or mini U-lock, and connected to an embedded ground anchor. Keep it short and out of the walkway.
For a permanent bench, the better system is to bolt the legs or mounting plates directly into a concrete pad or footing. I would use galvanized or stainless steel wedge anchors, sleeve anchors, or drop-in anchors rated for outdoor use. Mark the holes through the bench feet, drill into the concrete with a hammer drill, clean the dust out of the holes, then drive in the anchors. Secure the bench with tamper-proof bolts such as pin-head Torx, penta-head, or breakaway cone nuts. That makes removal much slower and less attractive to thieves.
If you are anchoring into a wooden deck, through-bolt the bench legs from underneath with large washers and lock nuts. Do not rely on short wood screws. If you cannot drill into the ground at all, a weighted base is usually better than a chain. Some benches have hollow bases that can be filled with sand or concrete, or you can bolt the bench to a heavy precast concrete pad or steel plate.
Before you install anything on public property, check with your local council or property owner. Unauthorized anchors, chains, or bolts on sidewalks, parks, or streets can be removed, and you may be responsible for the damage. For a private yard or business with a concrete pad, direct bolting is almost always the cleaner, safer, and more secure way to keep the bench from walking away.