Adolescent anemia is a global health problem. The ability of adolescents to prevent and treat anemia is directly related to the knowledge and attitudes they possess. This was to determine the effectiveness of education using websites and WhatsApp to increase the knowledge and attitudes of adolescents regarding anemia. This was a quasi-experimental study with a pretest-posttest with control group design. The research was conducted in March and April 2023 in Yogyakarta, Indonesia. The intervention was a website and WhatsApp education, while the outcome were knowledge and attitude measured at baseline and end line. The study sample was 61 experimental group student and 44 control groups determined by multistage sampling. The data were analyzed using Wilcoxon and MannWhitney tests using SPSS. The results showed that knowledge and attitude of adolescents both in the website and WhatsApp groups increased; knowledge scores in the website group baseline and end line were 90.87 and 95.4, respectively, with a change of 5,1%. While in the WhatsApp group, there were 85.6 and 94.8, with a change of 10.7%. The average pre-test and post-test attitude scores in the website group were 63.8 and 69.3 with a change of 8.6%, while in the WhatsApp group they were 83.3 and 89.1 with a change of 40.6%. The results showed that the increase in knowledge and attitudes in the WhatsApp group was significant: 1.02 (1.00–1.84) and 1.07 (91.01-0.96), respectively. As a conclusion, WhatsApp is more effective in increasing knowledge and attitude compared to website education.