Although parental care is generally rare among reptiles, nest-guarding occurs in some species and is usually attributed to defense against nest predation. Nest-guarding also occurs in the Tuatara (Sphenodon punctatus), but nest predation on rodent-free islands does not appear to be a significant threat to nesting success in this species. We studied a population of colonially nesting Tuatara on rodent-free Stephens Island, New Zealand, over four years and tested the hypothesis that female Tuatara guard their nests to defend them from excavation by conspecific females. We located 73 nests for which females could be assigned based on observations during oviposition. Nearly 25% of these nests were subsequently excavated by another female, but only 56% of the nests were guarded by the females that constructed them. We found a trend where guarded nests were less likely to be excavated than unguarded nests. Females were more likely to guard their nests, and guarded nests for longer, as the activity of other females on the date of oviposition increased. Nest-guarding in Tuatara appears to be adaptive in that it tends to reduce the likelihood of nest excavation by other females, but social interactions may affect females' propensity to guard, because guarding behavior was influenced by the activity of conspecifics at the time of oviposition.