There is little evidence on cognitions that are associated with emotional and behavioral problems in preschoolers during stressful events such as the COVID-19 pandemic. This article presents the initial development and validation of a caregiver-report instrument, the Preschooler Stressor-related Thoughts and Worries (PSTW) scale, developed during the COVID-19 pandemic. In 2020, caregivers from two countries reported on their child’s cognitions at baseline (T₀) and three months later (T₁; age 3–5 years; Australia: N = 559; United States: N = 346). Exploratory factor analysis (EFA) was conducted with the Australian sample at T₀ and confirmed with the U.S. sample at T₀. Exploratory factor analysis suggested a one-factor model including 10 items. Results of the confirmatory factor analysis failed to clearly support this structure (comparative fit index = .91, root mean square error of approximation = .11). Construct validity was supported by positive associations between PSTW scores and emotional and behavioral problems. Although the PSTW is a promising instrument to assess preschooler cognitions related to COVID-19, further investigation of its performance in other contexts (e.g., other countries, other stressful or traumatic events) is needed. (PsycInfo Database Record (c) 2022 APA, all rights reserved) (Source: journal abstract)