Yes—under the right conditions. An empty urban planter box can make a charming small water feature, but it should be treated as a water container, not just a pot. The first question is whether the box can hold water. Most planter boxes are designed to drain, so you will need to seal drainage holes or, more reliably, install a flexible pond liner with underlayment. Check seams, corners, and the rim; use a suitable caulk or liner adhesive where needed. If the planter is wood, metal, or ceramic, a liner protects the material and makes cleaning easier. Plastic may be UV-stable but can crack, so inspect it carefully.
Weight is critical. Water weighs about 1 kg per liter (8.34 lb per gallon), so even a small box gets heavy fast. Place it where the structure can support the load—especially on balconies, roof terraces, and decks. Add the pump, stones, and reservoir volume to the calculation. Keep it on a level, stable surface, and do not put it on a railing.
Plan for overflow and drainage. A water feature needs a safe overflow path for rain, or it will spill over the sides and damage nearby surfaces. If you seal the original holes, add an overflow fitting or leave a controlled weep path. Outdoors, mosquitoes are a concern: use a small pump to keep water moving, add mosquito dunks with Bti if needed, and avoid stagnant water. Clean the box thoroughly before use, especially if it held soil or fertilizer.
A small low-voltage pump can create a bubbler or spout. Use outdoor-rated electrical equipment, a GFCI-protected outlet, and keep cords away from water. If you do not want electricity, a still container water feature with stones and floating ornaments can work, but it needs frequent cleaning and mosquito control. Avoid fish in a tiny planter, because temperature and water quality swing too much. In freezing climates, drain and store the feature for winter.
Finally, check local rules, lease or balcony restrictions, and waterproofing concerns. With a liner, overflow, safe pump, and weight planning, an unused urban planter box can absolutely become a small water feature—just not a leaky, overloaded one.