FastJet is a C++ library providing a wide range of jet finding and analysis tools. It includes efficient native implementations of widely used 2→1 sequential recombination jet algorithms for pp and e⁺e⁻ collisions, as well as access to third-party jet algorithms via a plugin mechanism, including all current cone algorithms. FastJet also offers tools for jet substructure analysis, pileup noise estimation, jet area determination, and noise suppression in jets. The library includes core classes such as PseudoJet, JetDefinition, and ClusterSequence, which facilitate jet clustering and analysis. It supports various jet algorithms, including the longitudinally invariant kt algorithm, Cambridge/Aachen algorithm, and anti-kt algorithm. Plugins allow access to additional jet algorithms used in hadron and e⁺e⁻ collisions. FastJet provides functionality for jet substructure, including boosted heavy-object taggers, and tools for estimating and subtracting pileup and underlying-event noise. It also includes methods for determining jet areas and handling noise in jets. The library supports efficient clustering algorithms, with optimized implementations for large particle multiplicities. FastJet is distributed under the GNU General Public License (GPL) and includes extensive documentation, including a quick-start guide, core classes, and appendices detailing clustering strategies, user information in PseudoJets, and other functionalities. The library is designed for user extensibility, with support for plugins, selectors, and custom jet substructure tools. It also includes features for handling high-noise environments, such as background contamination in pp and heavy-ion collisions. FastJet provides tools for estimating and subtracting noise, as well as for analyzing jet substructure and identifying boosted objects. The library is widely used in collider experiments for jet finding and analysis.FastJet is a C++ library providing a wide range of jet finding and analysis tools. It includes efficient native implementations of widely used 2→1 sequential recombination jet algorithms for pp and e⁺e⁻ collisions, as well as access to third-party jet algorithms via a plugin mechanism, including all current cone algorithms. FastJet also offers tools for jet substructure analysis, pileup noise estimation, jet area determination, and noise suppression in jets. The library includes core classes such as PseudoJet, JetDefinition, and ClusterSequence, which facilitate jet clustering and analysis. It supports various jet algorithms, including the longitudinally invariant kt algorithm, Cambridge/Aachen algorithm, and anti-kt algorithm. Plugins allow access to additional jet algorithms used in hadron and e⁺e⁻ collisions. FastJet provides functionality for jet substructure, including boosted heavy-object taggers, and tools for estimating and subtracting pileup and underlying-event noise. It also includes methods for determining jet areas and handling noise in jets. The library supports efficient clustering algorithms, with optimized implementations for large particle multiplicities. FastJet is distributed under the GNU General Public License (GPL) and includes extensive documentation, including a quick-start guide, core classes, and appendices detailing clustering strategies, user information in PseudoJets, and other functionalities. The library is designed for user extensibility, with support for plugins, selectors, and custom jet substructure tools. It also includes features for handling high-noise environments, such as background contamination in pp and heavy-ion collisions. FastJet provides tools for estimating and subtracting noise, as well as for analyzing jet substructure and identifying boosted objects. The library is widely used in collider experiments for jet finding and analysis.