Background: The medical information about anemia prevention produced by health-care professionals is difficult to understand because of its complicated terminology. Purpose: To evaluate quantitatively for the first time the clarity of existing online information about anemia prevention in Japanese. Methods: On January 20, 2023, we used the keyword "anemia" to evaluate 200 websites, followed by use of the Patient Education Materials Assessment Tool for Printable Materials (PEMAT-P) and the Clear Communication Index (CCI) to evaluate understandability and actionability. The PEMAT-P and CCI were both evaluated as excellent in terms of understandability and actionability, with cutoff values of 70% or higher and 90% or higher, respectively. We also evaluated readability using jReadability and quality using the Journal of the American Medical Association (JAMA) Benchmark Criteria and The Global Quality Scale (GQS). Results: The PEMAT-P scores for understandability and actionability exceeded the cutoff value in 59 (29.5%) and 36 cases (18.0%), respectively, while the CCI exceeded the cutoff value in two cases (1.0%). With respect to web pages, nine (4.5%) were considered "readable" and 96 (48.0%) "neutral." Discussion: These findings suggest that websites on anemia in Japanese have poor "understandability," "actionability," and "readability," and are therefore inaccessible to the general public. Translation to Health Education Practice: Improving existing anemia websites using indicators such as the PEMAT-P could improve behaviors related to anemia prevention.