Once China seized control of Paracel & Spratly islands from Vietnam in 1970s and 80s, it exercised ‘restraint’ viz à viz its ‘calculative strategy’ in later decades and ratified UNCLOS and signed the ‘Declaration on the Conduct of Parties in the South China Sea (SCS) to prevent any change of feature of the Sea. In late 2000s, China became relatively assertive in the SCS and declared it its ‘core interest’, thus began developing the partially submerged reefs into artificial islands and militarizing them. This paper explores the factors responsible for China’s assertiveness in this long-standing, multi-party dispute over the SCS employing broader framework of realist lens. Data is mostly collected from primary and secondary sources whereas the data analysis is based on qualitative content analysis with more focus of its theory testing approach manifest analysis. China’s ambitions fit into the Hegemonic Transition Theory as it is vociferously challenging the US hegemony in the SCS and aiming at consolidating its ‘regional hegemony’. The US has increased its Freedom of Navigation Operations in the Sea, which is a natural hegemonic response. China has justified its actions in the SCS based upon its historical claims on disputed islands and rejected the United States presence based upon its non-resident status. Motivation behind China’s actions in the SCS is more security oriented rather than economic, as China aims at consolidating its grip on the SCS routes from where most of the its energy imports pass.