Abstract Introduction In emancipation policy, the ‘I’ is often added to the LGBTI+-acronym. Yet it is still unknown to what extent this term is known and understood by the general population, and what attitudes toward intersex (people) are. In the current study, we describe the construction and development of the Intersex Knowledge and Attitudes Scale (IKAS), as well as the results from this baseline measurement in the Dutch and Flemish population. Methods The IKAS was developed after a carefully documented stepped process of literature scan, expert consultations, and pretesting. A representative sample of 2,003 respondents (1,000 Dutch and 1,003 Flemish) participated in the study. To determine the dimensionality of the scale, we performed EFA and CFA with measurement invariance constraints. We also performed a baseline measurement of knowledge and attitudes towards intersex people in the general population, and determined predictors of attitudes toward intersex people in regression analyses. Results Two-thirds of the sample had no (correct) knowledge of the term intersex. Even among the intersex respondents, only one person was aware of the term intersex. The average attitude was moderately positive to neutral. Having a female gender or birth sex, an advanced level of education, lower importance of religion, belonging to a minority group, more tolerant gender beliefs, and preexisting knowledge on the topic all resulted in more positive attitudes. Conclusions Both in Flanders and the Netherlands there still is room for improvement in terms of knowledge and attitudes towards intersex people. More knowledge may lead to more understanding, so that intersex people can be more open. Second, it is recommended to challenge beliefs of sex and gender as binary categories, because people who have less issues with people who do not entirely fall in these categories, also have more accepting attitudes toward intersex people. Key messages • It is vital to include intersex in monitoring studies of population attitudes, to be able to genuinely incorporate the ‘I’ in LGBTI+ in emancipation policies. • To increase acceptance of intersex people in society, knowledge about intersex in the general population should be expanded, and acceptance of people who do not fit into the binary should be promoted.