Anaemia is one of the major public health problems worldwide and is associated with morbidity and mortality. Anaemic adolescent girls are at high risk of having physical and cognitive functional disorders. According to the WHO global database, anaemia affects 46% of school children globally. Because of health and socioeconomic problems, the prevalence of anaemia is higher in developing countries than in developed ones with 50% in Africa where children, adolescent girls and pregnant women are the most vulnerable groups to anaemia. In Rwanda, there is little national data on anaemia and its likely causes amongst adolescent girls. This study aimed to assess the extent of anaemia and associated risk factors among adolescent girls aged between 10-19 years old attending St Therese Clinic located in Eastern province of Rwanda. A cross sectional study design was conducted at St Therese Clinic and included 231 adolescent girls aged between 10-19 years old who were conveniently selected from the consented participants. The hemocue apparatus which uses a modified azide-methaemoglobin reaction was used to measure Haemoglobin (Hb) level in the whole blood. Pre-tested structured questionnaires were used to collect data on anticipated risk factors. The prevalence of anaemia was found to be 29% among adolescents girls. Among 67 anaemic girls (29%), 45 girls (19.47%) had mild anaemia while 18 girls (7.79%) had moderate anaemia and 4 grils (1.73%) had severe anaemia. It was found that malaria, menarche, educational status of their parents and meal intake frequency are significantly associated with anaemia, p value <0.05. Anaemia is highly prevalent in adolescent girls. Thus, policy makers has to prevent and control anaemia in this particular vulnerable group through but not limited to Iron/folic acid supplement, malaria eradication and promoting nutrition education programme targeting adolescent girls and their mothers, heads of primary and secondary schools mainly on the utilisation of easily available and affordable iron rich diet.