Mothers of children diagnosed with cancer have been found to be at a heightened risk for post-traumatic stress symptoms (PTSS). In an effort to identify a potential buffer, hardiness was examined as a protective factor for PTSS among mothers of children that were diagnosed with cancer in the past 2 weeks. Using a prospective design, mothers completed measures of PTSS and hardiness at the time of their child's cancer diagnosis and then again at 6- and 12-months post-diagnosis. Random effects regression analyses revealed that mothers who scored high on hardiness were less likely to experience PTSS after controlling for the effect of time. PTSS cluster-specific relations with hardiness were also examined, which revealed that mothers who scored high on hardiness experienced fewer avoidance/numbing symptoms at the time of their child's diagnosis of cancer and across 12 months; whereas mothers who scored low on hardiness tended to experience more avoidance/numbing symptoms at the time of their child's diagnosis. However, these symptoms declined gradually over the course of 12 months. The present findings support examining hardiness further as a buffer against specific PTSS clusters and exploring options for identifying and treating mothers of children with cancer that may be at risk for PTSS.