The late Paleocene to early Eocene was a prolonged period of global warming punctuated by abrupt intervals of rapid temperature rise. These hyperthermal events, especially including the Paleocene-Eocene thermal maximum, are characterized by negative carbon isotope excursions (CIEs), which signify massive input of ¹³C-depleted carbon. A widely-discussed mechanism to explain such carbon injection is destabilization and degassing of methane hydrates in marine sediment. Although large amounts of methane hydrate deposits should have existed during the late Paleocene and early Eocene, evidence has been limited. This study documents late Paleocene and early Eocene siderite-dominated carbonate concretions hosted in turbidites of the Dukla Nappe, Outer Carpathians, Poland. These concretions have δ¹³C ranges attributable to formation in methanogenic environments. Furthermore, grain-to-grain relationships and preserved sedimentary fabrics indicate authigenic formation prior to compaction. Given that they were deposited in sufficient water depth to host stable hydrates despite elevated ocean temperatures, these concretions provide supporting evidence of active methanogenesis and the accumulation of methane hydrates during the late Paleocene and early Eocene.