Water retention curves are essential for the parameterization of soil water models such as HYDRUS. Although hydraulic parameters are known for a large number of mineral and natural organic soils, our knowledge on the hydraulic behavior of cultivated Histosols is rather limited. The objective of this study was to derive characteristic water retention curves for a large cultivated peatland with lettuce (Lactuca sativa L.) and vegetable farming in southern Quebec, Canada. A comparison showed that the van Genuchten model fits better to the water retention data obtained with a Tempe pressure cell experiment than the Groenevelt–Grant model in terms of residual sum of squares; however, the difference in performance was quite small due to the high number of iterations used for fitting. Finally, an agglomerative cluster analysis of 85 peat samples allowed us to define two distinct water retention curves, where the first water retention curve described samples of relatively shallow (<150 cm) Histosols with an organic content <0.89 and a bulk density >0.3 g cm−3, and the second curve characterized samples of the deepest (depth 150–230 cm) Histosols with an organic content of up to 0.97 and a bulk density >0.3 g cm−3, which are the soils that suffered a more dramatic transformation as a result of agriculture. This characterization allows for a multitude of applications, including parameterization of the HYDRUS model for soil water movement, and presents an essential tool for the optimization of water management in cultivated peatlands.