The FastJet user manual (version 3.0.1) provides a comprehensive guide to the FastJet C++ package, which offers a wide range of jet finding and analysis tools. It includes efficient implementations of widely used 2→1 sequential recombination jet algorithms for both hadron-hadron and $e^+e^-$ collisions, as well as access to third-party cone algorithms through a plugin mechanism. The manual covers core classes such as PseudoJet, JetDefinition, and ClusterSequence, and discusses various jet algorithms, including the longitudinally invariant $k_t$ algorithm, Cambridge/Aachen algorithm, Anti-$k_t$ algorithm, and generalized-$k_t$ algorithms. It also explains how to use FastJet for jet substructure analysis, background estimation, and jet area determination. Additionally, the manual provides instructions for setting up and using FastJet, including a quick-start guide and detailed examples. The package aims to facilitate jet finding and analysis in high-energy physics experiments, particularly at the Large Hadron Collider (LHC).The FastJet user manual (version 3.0.1) provides a comprehensive guide to the FastJet C++ package, which offers a wide range of jet finding and analysis tools. It includes efficient implementations of widely used 2→1 sequential recombination jet algorithms for both hadron-hadron and $e^+e^-$ collisions, as well as access to third-party cone algorithms through a plugin mechanism. The manual covers core classes such as PseudoJet, JetDefinition, and ClusterSequence, and discusses various jet algorithms, including the longitudinally invariant $k_t$ algorithm, Cambridge/Aachen algorithm, Anti-$k_t$ algorithm, and generalized-$k_t$ algorithms. It also explains how to use FastJet for jet substructure analysis, background estimation, and jet area determination. Additionally, the manual provides instructions for setting up and using FastJet, including a quick-start guide and detailed examples. The package aims to facilitate jet finding and analysis in high-energy physics experiments, particularly at the Large Hadron Collider (LHC).