In J. R. R. Tolkien's fictional universe of Middle-earth, the Fens of Serech were a series of marshes in the Pass of Sirion where many Battles of Beleriand saw much warfare.

Formed by the confluence of the rivers Sirion and Rivil, the Fens were trapped in a wedge formed by the Ered Wethrin and the highlands of Dorthonion.

During the Dagor-nuin-Giliath the Orcs of Morgoth were caught in the Fens for ten days, where they were being destroyed by the enraged Sons of Fëanor.

During the Dagor Bragollach, Finrod Felagund was caught in the Fens, and would have died but for the bravery of Barahir.

During the Nirnaeth Arnoediad, Huor and Húrin with the remaining Men of Dor-lómin formed a living wall across the Fens of Serech to guard the withdrawal of King Turgon of Gondolin, who took most of the surviving Noldor with him. There they were all slain, except for Húrin, who was captured.

Like the rest of Beleriand the Fens were destroyed during the War of Wrath.