This study aims at modeling the self-reported test-taking motivation items in PISA and TIMSS Advanced studies for Swedish students using an IRT approach. In the last two cycles of the assessments, six test-specific items were included in the Swedish student questionnaires to evaluate pupil’s effort, motivation and how they perceived the importance of the tests. Using a Multiple-Group Generalized Partial Credit model (MG-GPCM), we created an IRT motivation scale for each assessment. We also investigated measurement invariance for the two cycles of PISA (i.e., 2012 and 2015) and of TIMSS Advanced (i.e., 2008 and 2015). Results indicated that the proposed scales refer to unidimensional constructs and measure reliably students’ motivation (Cronbach’s alpha above 0.78). Differential item functioning across assessment cycles was restricted to two criteria (RMSD and DSF) and had more impact on the latent motivation scale for PISA than for TIMSS Advanced. Overall, the test-taking motivation items fit well the purpose of a diagnostic of test-taking motivation in these two surveys and the proposed scales highlighted the slight increase of pupils’ motivation across the assessment cycles.