Lago di Posta Fibreno is an Italian lake located in Posta Fibreno, a municipality of the Province of Frosinone, Lazio. At an elevation of 288 m, its surface area is 0.287 km<sup>2</sup>. The lake is in a karstic terrain and rich in underwater springs.
The lake of Posta Fibreno is home to the endemic Fibreno trout (Salmo fibreni), along with the more widespread Mediterranean trout (Salmo cettii).