This study examined the relevance of the Goal Instability Scale-Portuguese Adaptation (GIS-P) on the vocational identity and career certainty of 375 10th-, 11th-, and 12th-grade Portuguese high schoolers. After translating the GIS-P, confirmatory factor analysis was used to examine the factorial unidimensionality and reliability of the construct. A multitrait-monomethod correlation matrix demonstrated convergent and divergent validity with personality, career, and social constructs. As expected, the GIS-P was a stronger predictor of vocational identity than global self-esteem and differentiated those students with postsecondary plans from those who were uncertain about their plans. This research supports the cultural relevance of the goal instability construct and of motivational determinants of academic and career adjustment with Portuguese students.