The study aims to determine the prevalence of self-harm (SH) and related psychosocial factors in a large sample of Portuguese adolescents. A total of 1,713 pupils, aged 12 to 20 years, completed an anonymous questionnaire in a school setting. In all, 7.3% reported at least 1 episode of SH: rates were 3 times higher for females than males. Almost half reported repeated SH, most commonly self-cutting. Anxiety, depression, and substance abuse were linked to SH, and particularly repeated SH. Anxiety, trouble with the police, and exposure to SH or suicide of others, were independently associated with SH in both genders. These findings indicate that SH is a public health concern in Portugal as in other European countries.