We introduce and investigate the philosophical concept of 'speciesism' -the assignment of different moral worth based on species membership -as a psychological construct. In five studies, using both general population samples online and student samples, we show that speciesism is a measurable, stable construct with high interpersonal differences, that goes along with a cluster of other forms of prejudice, and is able to predict real-world decision-making and behavior. In Study 1 we present the development and empirical validation of a theoretically driven Speciesism Scale, which captures individual differences in speciesist attitudes. In Study 2, we show high test-retest reliability of the scale over a period of four weeks, suggesting that speciesism is stable over time. In Study 3, we present positive correlations between speciesism and prejudicial attitudes such as racism, sexism, homophobia, along with ideological constructs associated with prejudice such as social dominance orientation, system justification, and right-wing authoritarianism. These results suggest that similar mechanisms might underlie both speciesism and other well-researched forms of prejudice. Finally, in Studies 4 and 5, we demonstrate that speciesism is able to predict prosociality towards animals (both in the context of charitable donations and time investment) and behavioral food choices above and beyond existing related constructs. Importantly, our studies show that people morally value individuals of certain species less than others even when beliefs about intelligence and sentience are accounted for. We conclude by discussing the implications of a psychological study of speciesism for the psychology of human-animal relationships. (PsycINFO Database Record (c) 2019 APA, all rights reserved).