PurposeThe development of health literacy skills is of paramount importance especially for students, as it is associated with their wellbeing and academic success. Assessing students' health literacy is necessary for its advancement. This study comes to fill the need for a valid multidimensional health literacy assessment instrument for secondary school students.Design/methodology/approachInitially, the authors defined the conceptual framework within which the items of this instrument were generated. Its final form was obtained through three pilot studies. A test-retest followed and, finally, a nationwide survey was conducted on Greek seventh-, ninth- and tenth-grade students (N = 2,800).FindingsA 37-item instrument, the Health Literacy Assessment for Secondary School Students (HeLiASeSS), was developed which includes the following nine dimensions/skills of health literacy: “access to”, “understand” and “evaluate” (health information), “functional communication”, “interactive communication” and “critical communication”, “relying on health information”, “self-efficacy regarding health matters” and “intention for active citizenship regarding health matters”. HeLiASeSS proved to have good stability (ICC = 0.943) and high internal consistency reliability (a = 0.903).Originality/valueHeLiASeSS offers the possibility of a reliable and valid assessment of secondary school students' health literacy skills providing a multidimensional evaluation of this construct and is expected to be useful in interventions aiming at promoting this type of literacy.