Pain that moves from the buttocks to one or both legs usually stems from irritation of the sciatic nerve, explains WebMD. This medical condition is termed "sciatica."
The sciatic nerve is the body's widest and longest nerve, reports HealthLine, running from the lower back, across the buttocks and down each leg. It consists of nerve roots that protrude into the lower back from the spinal cord and contains sensory and motor neurons that enable feeling and movement in the legs. Sciatica may result from a bulging or ruptured disc, narrowing of the spinal canal, bony growths or a pinched nerve, notes WebMD. Tumors may cause sciatica in rare cases.