Racially and ethnically diverse populations from minoritized backgrounds are often exposed to research methodologies that amplify structural racism and negate their sociocultural reality. Although cross-cultural validation of measures is considered a requisite step to multigroup comparisons, researchers apply measures validated and standardized in the dominant White culture to under-researched populations (without assessing measurement equivalence first). Seeking to align with calls for more culturally sensitive measures, an argument is made to leverage youth participatory approaches in the cross-cultural validation of study instruments. Using an illustrative case study, this paper describes how 16 youth researchers (predominantly Hispanic) partnered with an academic team to examine the validity of the Prosocial Behavior Scale. A new tool, the Key Informant Validity Index, is introduced to determine if items maintain adequate levels of content validity when applied to populations that differ from the original norming and validation study samples. Youth researchers rated items on relevance, clarity and coverage, and use a guided protocol to conduct cognitive interviews with 32 youth participants. Recognizing the inherent challenges of reducing hierarchal power dynamics within youth-adult relationships and/or alleviating insider/outsider tensions across lines of cultural difference, an intentional focus is placed on naming key strategies that facilitated the collaboration process.