Short answer: yes, a park bench can be placed on grass, but “safe” depends on the soil, weather, bench weight, and how much traffic the bench gets. For a light decorative bench in a dry backyard, grass alone may be fine for a while. For a public park bench or a heavy bench that people will use daily, a solid foundation or at least a prepared base is strongly recommended.
The main problem with grass is that it is a living, soft surface. After rain or irrigation, soil underneath becomes soft and the bench legs can sink. Even if the bench does not sink evenly, it can start to lean. A leaning bench feels unstable, and over time the legs can rust or rot where they stay damp. On a slope, placing a bench directly on grass is riskier because shifting soil and water runoff can make it slide or tip.
That said, you do not always need a full concrete slab. A practical middle ground is to create a simple foundation. Dig out the grass and topsoil under each leg or under the whole bench footprint, add a few inches of compacted gravel, then set concrete pavers, stone pads, or a pre-cast base on top. The bench legs can then be anchored to the pads with brackets or heavy-duty spikes. This keeps the bench level, prevents sinking, and improves drainage. For public spaces, a more robust foundation may be required by local codes, including concrete footings or a paved pad.
If you really want to place a bench directly on grass, choose a bench with wide, flat feet or skids instead of narrow legs. Place it on firm, level ground, and check it after heavy rain. Push down on each corner to feel for rocking. If it rocks, digs in, or wobbles, move it or add a base. Never install a park bench on grass on a steep slope without anchoring it, because it can become a tipping hazard.
So, is it safe? Yes, if you match the installation to the situation. For light use, dry ground, and a wide-footed bench, grass may be acceptable. For permanent, public, or heavy-use benches, a solid foundation is the safer choice—and it extends the life of the bench.