Nitrogen pollution in water bodies originated from various anthropogenic activities remains a growing concern in view of its significant environmental impacts. This highlights the necessity of efficient nitrogen treatment systems. In biological treatment, nitrogen species can be removed through nitrification and denitrification processes. Particularly, denitrification process occurs under anaerobic conditions where nitrates are transformed to nitrogen gas through a series of reduction reactions in the presence of carbon sources as electron donors. This book chapter explores the role of polyhydroxyalkanoates (PHAs) as carbon sources for denitrification in wastewater treatment since conventional carbon substrates present sustainability issues. PHAs, biopolymers with diverse structures, offer a promising alternative attributed to their biocompatibility and biodegradability. The utilization of PHAs as endogenous and exogenous carbon supply to enhance denitrification, reduce operational costs, and minimize environmental impacts is discussed. Albeit the challenges in high production cost and strict conditions for PHAs production that hinder the widespread use of PHAs in wastewater treatment, ongoing research and advancement in PHAs production and application hold promise for addressing nitrogen pollution promoting sustainable and environmentally friendly denitrification processes.