Background: New approaches for chemomechanical caries removal require effective materials with antibacterial properties for removal of infected dentin. Apacaries gel is a newly developed material comprised polyphenol from mangosteen extracts and papain mixed in gel preparation. Aim: This study evaluated the antibacterial effects of Apacaries gel on Streptococcus mutans in vitro. Materials and methods: Mangosteen pericarp powder was extracted. The amount of phenolic compounds was determined using the Folin-Ciocalteu method. The time-kill kinetics were investigated. Mangosteen extract and papain were mixed with gel base to develop Apacaries gel. The inhibition zone of the Apacaries gel was determined using agar well diffusion methods. Results: The mangosteen pericarp extract, which contains α-mangostin, was active against S. mutans strain ATCC25175. The time-kill kinetics curve showed that applying 1 mg/ml of mangosteen extract can reduce S. mutans by 50% within approximately 5 seconds; after this reduction, the bacterial count rapidly dropped to 0 within 60 seconds. Using mangosteen extract and papain mixture gel preparation resulted in a larger inhibition zone than using the mangosteen extract gel or papain gel separately. Conclusion: Apacaries gel can effectively inhibit S. mutans strain ATCC25175. Apacaries is capable of S. mutans inhibition better than both mangosteen extract or papain separately. The Antibacterial Effects of Apacaries Gel on Streptococcus mutans: An in vitro Study. Int J Clin Pediatr Dent 2014;7(2):77-81.