Polymers with high molecular weights have superior properties, such as enhanced impact and chemical resistance. While these properties can be achieved by converting a thermoplastic into a thermoset, this can prevent polymers from further processing as thermosets cannot be melted and moulded without damaging their internal structure. Therefore, increasing the molecular weight of a polymer without losing the ability to process it is of utmost importance. Polyethylene (PE), the most commonly produced plastic in the world, is comprised of strong C–C and C–H bonds, which makes its controlled chain extension challenging to achieve. Herein, we report a novel, solution-based method for the modification of PE chains using commercially available, low-cost organic peroxides in solvents such as dichlorobenzene and tert-butylbenzene. To the best of our knowledge, this is the first solution-based methodology for PE modification through the incorporation of long chain PE branches by using organic peroxides. The effects of the modification reactions were extensively investigated using rheology, differential scanning calorimetry, small/wide angle X-ray scattering, size exclusion chromatography and NMR spectroscopy, and model studies were performed with n-dodecane to confirm the formation of branched moieties. The enhanced mechanical properties of the materials were demonstrated using rheology, where the modified polymers show significantly increased stiffness and higher viscosities. This is attributed to reactions between the PE chains to form branched structures, thus increasing both the molecular weight of the feedstock and the number of entanglements within the polymer microstructure. This methodology enables the properties of PE to be tailored, providing a shortcut for the development of new PE grades and formulations as its applications continue to grow in developing technologies such as 3D printing, artificial joints and soft robotics.