Caries could be minimized by reducing plaque accumulation with an antiseptic mouthwash. However, chlorhexidine 0,2%, the gold standard mouthwash, is known to have side effects. This has led to the innovation of an alternative herbal-based mouthwash that has antibacterial properties; one of the possible natural material is red dragon fruit peels. This study was aimed to prove the effect of red dragon fruit peel extract as mouthwash prepared in several concentrations to inhibit the growth of Streptococcus mutans. This was a quasi-experimental study with the post test control group design. There were six sample groups, namely mouthwash with extract concentration of 6.25%, 12.5%, 25%, and 50%, negative control (aquadest), and positive control (chlorhexidine 0.2%). The antibacterial assessment was done by using the well diffusion method calculating the inhibition zone formed around the wellbore. Data were analyzed by using the Kruskal Wallis test and the Mann Whitney Post Hoc test. The results showed that the average inhibition zones formed by extract mouthwash of 6.25%, 12.5%, 25%, 50% were 3.18 mm, 5.13 mm, 7.30 mm, and 11.01 mm respectively, and by positive control was 14.65 mm. There were significant differences between each treatment group. All concentrations of red dragon fruit peel extract in mouthwash could inhibit effectively the growth of Streptococcus mutans although not as effective as the positive control. In conclusion, red dragon fruit peel extract mouthwash could inihibit the growth of Streptococcus mutans.Keywords: mouthwash; red dragon fruit peel; Streptococcus mutans; antibacterial effect  Abstrak: Karies dapat diminimalisasi dengan mengurangi akumulasi plak menggunakan obat kumur antiseptik. Obat kumur chlorhexidine 0,2% merupakan obat kumur baku emas namun memiliki efek samping. Hal tersebut memunculkan inovasi untuk mencari alternatif obat kumur berbahan dasar herbal yang memiliki sifat antibakteri; salah satunya ialah kulit buah naga merah. Penelitian ini bertujuan membuktikan pengaruh ekstrak kulit buah naga merah dalam obat kumur dengan beberapa konsentrasi untuk menghambat pertumbuhan Streptococcus mutans penyebab karies. Jenis penelitian ialah quasi experimental dengan desain post test only group control design. Terdapat enam kelompok sampel yaitu obat kumur konsentrasi ekstrak 6,25%, 12,5%, 25%, 50%, kontrol (-) akuades, dan kontrol (+) chlorhexidine 0,2%. Uji antibakteri menggunakan metode difusi sumuran dengan menghitung zona hambat yang terbentuk di sekitar lubang sumuran. Uji statistik menggunakan Kruskal Wallis  dan uji post hoc Mann Whitney. Hasil penelitian mendapatkan rerata zona hambat yang terbentuk dari obat kumur konsentrasi ekstrak 6,25%, 12,5%, 25%, 50% ialah 3,18 mm, 5,13 mm, 7,30 mm, 11,01mm, dan kelompok kontrol(+) ialah 14,56 mm. Perbedaan bermakna terdapat pada setiap kelompok perlakuan. Setiap konsentrasi ekstrak kulit buah naga merah dalam obat kumur berpengaruh menghambat pertumbuhan bakteri Streptococcus mutans, namun belum sebaik kontrol (+). Simpulan penelitian ini ialah ekstrak kulit buah naga merah dalam obat kumur berpengaruh menghambat pertumbuhan bakteri Streptococcus mutans.Kata kunci: obat kumur; kulit buah naga merah; Streptococcus mutans; efek antibakteri