This study aimed to identify microorganisms and analyze the nutrient content of bioactivator derived from fermented papaya, as well as assess the effect of bioactivator administration on the composting process and the chemical properties. The study process was conducted in Telang Sari Village, Indonesia, from May–July 2025. The materials used were rice straw, biochar, cow dung, fermented papaya as a bioactivator, and clean water. A completely randomized design (CRD) was used with four treatments and three replications, including P0 (0 L bioactivator), P1 (0.25 L), P2 (0.5 L), and P3 (0.75 L). The variables observed were the type of microorganisms and nutrient content of bioactivator, composting temperature, and the chemical properties of the compost. The results showed that fermented papaya contained isolates (Rhizopus oryzae and Aspergillus niger), 0.890% N, 0.244% P, and 0.250% K, which were used as bioactivator. The addition of bioactivator increased compost temperature and cation exchange capacity (CEC). The temperatures of P3 (32.3 ± 0.6 °C) and P2 (31.7 ± 0.6 °C) were higher and significantly different (P < 0.05) compared to P1 (31.0 ± 0.0 °C) and P0 (31.0 ± 0.0 °C) in the first week. Similarly, the CEC values of P3 (30.28 ± 1.38 cmol/kg) and P2 (31.06 ± 5.19 cmol/kg) were higher and significantly different (P < 0.05) compared to P1 (21.50 ± 2.82 cmol/kg) and P0 (20.42 ± 3.56 cmol/kg). The use of bioactivator from fermented papaya facilitated biocomposting while improving the compost quality.