Start by checking whether the crack is cosmetic or structural. A hairline crack or a small chip in a ceramic urban planter box is usually repairable. If the pot is splitting apart, has loose pieces, or the crack runs through the base, it is safer to replace it or use it as a decorative outer pot with a plastic liner inside.
Remove the plant and most of the soil so you can work on the crack easily. Clean the crack with a stiff brush, then wipe it with rubbing alcohol or mild soapy water. Rinse only if needed, and let the planter dry completely. Repair products will not bond well to damp or dirty ceramic.
For a hairline crack, apply a thin bead of waterproof ceramic adhesive or two-part epoxy and work it into the crack. Press the edges together if possible, and hold them with tape or a clamp until the adhesive sets. For a wider crack or missing chip, use epoxy putty or a ceramic repair compound. Fill the gap slightly above the surface, let it cure, then sand it smooth.
If the crack is on the inside and the planter holds soil, reinforce the repair with a patch of fiberglass mesh embedded in epoxy. This adds strength and helps prevent the crack from spreading. Avoid hot glue and standard superglue for outdoor planters because they are not durable or waterproof enough.
After the repair cures, apply a waterproof ceramic or masonry sealer if the planter will stay outside. Let it cure fully before planting, and rinse the inside well. Check that the drainage holes are open. If the repaired area may still leak, line the planter with heavy plastic or use a plastic nursery pot as an insert. Replant only after the repair is completely dry and stable. To reduce future cracks, avoid freeze-thaw damage, do not overfill the planter, and move it carefully when it is full.