The combination of sonochemical irradiation and reversible addition–fragmentation chain transfer (RAFT) ab initio emulsion polymerization of methyl methacrylate (MMA) in water without exogenous initiator or surfactant is reported for the first time. The impact of ultrasound power (P) and frequency (f), of the reaction medium temperature, of solid content (τs), and of the initial molar ratio [MMA]0/[RAFT agent]0 on the kinetic and the control of the sono-activated RAFT ab initio emulsion polymerization of MMA was highlighted. The process in the presence of surfactant provides well-defined poly(methyl methacrylate) (PMMA) latexes in terms of number-average molecular weight (2600 ≤ Mn ≤ 8300 g mol–1), dispersity (1.13 ≤ Đ ≤ 1.23), and ω-chain-end functionality as well as hydrodynamic diameter (14 ≤ Dh ≤ 37 nm), polydispersity index in size (0.109 ≤ pdi ≤ 0.284), and colloidal stability of the latex particles (26 weeks). The implementation of the optimal conditions (f = 490 kHz, P = 40 W, T = 60 °C, [MMA]0/[RAFT agent]0 = 50/1, τs = 5 wt %) in the absence of surfactant allowed the synthesis of a well-defined exogenous initiator-free and surfactant-free PMMA latex. This environmentally friendly process relies on hydroxyl radicals generated from sono-irradiation of the continuous phase of the emulsion, on an optimal dispersion of multiphasic system via ultrasound, and on the control of the growth of macromolecular chains by the thiocarbonylthio-containing RAFT agent.