The present study examined the dimensionality and the measurement invariance of the Multicultural Ideology Scale (MCI; Berry & Kalin, 1995), and mean differences across different cultural groups within the multilingual, multicultural context of Luxembourg. Luxembourg is a unique context to study attitudes towards diversity because 47% of the citizens are non-nationals, and minority and majority are increasingly difficult to define. Our sample included 1,488 participants from diverse ethnic backgrounds who completed the survey in German, French or English. In contrast to previous findings, our analyses on responses to the MCI scale revealed a two-dimensional structure, distinguishing between positive and negative attitudes towards multiculturalism. The factor structure was partially invariant across ethnocultural groups: Configural and metric invariance were established across natives and non-natives and different language versions. Scalar invariance was only established across gender groups. Natives and male participants reported more negative attitudes towards multiculturalism. We discuss the importance of assessing measurement invariance and provide recommendations to improve the assessment of psychological multiculturalism.