Mobile　ad　hoc　networks　(MANETs)　are　surrounded　by　various　vulnerabilities　and　attacks　due　to　open　medium,　dynamic　topology,　limited　energy,　and　absence　of　central　control.　In　MANET,　each　attack　has　different　behavior　and　aftermaths.　DoS　attack　is　one　of　the　serious　attacks　in　MANET,　which　disturbs　the　normal　routing　process.　Jellyfish　(JF)　attack　is　a　type　of　DoS　attack　in　MANET,　whose　dynamic　behavior　makes　it　quite　difficult　to　detect　such　attacks.　In　this　paper,　we　propose　a　technique　to　detect　the　jellyfish　attack　in　MANET.　The　proposed　technique　combines　the　authentication　and　trustworthiness　of　nodes　and　the　kNN　algorithm　for　the　identification　of　jellyfish　attacks　in　which　each　and　every　node　calculates　the　primary　and　secondary　trust　values　to　detect　the　attacking　node　by　the　recommendation　of　neighboring　nodes　and　trust　metrics.　The　kNN　algorithm　separates　the　jellyfish　nodes　from　other　legitimate　nodes　based　on　the　differences　in　their　behavior.　The　proposed　technique　would　then　pick　reliable　nodes　by　the　hierarchical　trust　assessment　property　of　nodes　to　perform　packet　routing.　The　experiment　shows　that　the　proposed　technique　could　decrease　delay　and　increase　the　throughput　of　the　network　by　avoiding　jellyfish　nodes.　©　2021