© 2017 American Psychological Association. Mothers of children diagnosed with cancer have been found to be at a heightened risk for posttraumatic 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 postdiagnosis. 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, but 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.