PMMA is one of the most used polymers for optical applications, due to its well-known optical properties and low-cost. PMMA/fumed silica nanocomposites were synthesized by in situ polymerization under sonication to produce optical materials using two types of silica, a PDMS surface-modified and an unmodified one. Silica content and sonication amplitude effects on nanocomposites properties were studied by factorial experimental designs. Nanocomposites retained the high transparency of pristine PMMA, especially at lower levels of silica and sonication. Rheological analysis indicated better dispersion of the unmodified silica in PMMA. Dispersed silica in the PMMA nanocomposites decreased the PMMA refractive index by 0.012, making PMMA/fumed silica suitable for the cladding layer of PMMAcore waveguides, resulting in the total reflectance phenomenon for light guiding. Therefore, PMMA/fumed silica nanocomposites provide promising materials for polymer optical devices, such as optical fibers and panels, optical sensors and biosensors, photonic platforms, daylighting, and multi-touchscreen displays.