Although parenting stress has been identified as one of the most important and highly acceptable targets for postadoption psychological intervention, knowledge regarding the modifiable factors that contribute to explaining this outcome among adoptive parents remains scarce. This study aimed to explore whether and to what extent adoptive parents’ mindfulness, psychological flexibility, and self-compassion contribute to explaining parenting stress and to analyze whether this contribution varies according to children’s age, time passed since the adoptive placement, and the parents’ gender. Cross-sectional data from 302 Portuguese adoptive parents with children between 1 and 17 years old were collected online through self-response questionnaires. Controlling for a wide range of child-, adoption-, and parent-related variables, lower levels of mindfulness, psychological flexibility, and self-compassion were associated with higher levels of parenting stress. These associations were not moderated by children’s age, time passed since the adoptive placement, or the parents’ gender. The final regression model explained 50% of the variance in parenting stress. These results allow us to reflect on new guidelines for both preventive and remedial interventions with adoptive parents, given the apparent added value of promoting these psychological resources.