Device for filtering a liquid, particularly for filtering water in hydraulic circuits of heating systems, comprising: - a main body having a first duct for the inlet of the liquid to be filtered and a second duct for the outlet of the filtered liquid, said main body further comprising a hollow cylindrical portion; - a filter cartridge housed in said hollow cylindrical portion so as to divide a containment volume defined by said hollow cylindrical portion into a first volume, in fluid communication with said first duct, and a second volume, in fluid communication with said second duct; - valve means (116, 145) for controlling the flow of the liquid from said first duct to said first volume, said valve means (116, 145) comprising a valve seat interposed between said first duct and said first volume and a shutter member movable with respect to said valve seat between a closed position and an open position, in which it interrupts and allows, respectively, the flow of the liquid from said first duct to said first volume through said valve seat; - actuator means for controlling the movement of the shutter member between said closed position and said open position, wherein said actuator means comprise a rod, which is operatively associated with the shutter member and is shiftable along an axis of translation coinciding with its longitudinal axis, an actuator body, which is mounted on the main body and is configured to guide said rod in its translational movement of along said axis of translation, and a knob configured to be driven into rotation by a user to move said rod, and a shaft drivingly connected for rotation with said knob; and - first sealing means arranged inside the actuator body; characterized in that said rod is locked with respect to rotation around said axis of translation and has, at a first end portion thereof facing away from the shutter member, a threaded cavity; in that the shaft comprises a threaded portion engaging in said threaded cavity of the rod, in such a way that the rotational movement of the knob, and therefore of the shaft, in one direction or the other, is converted into a translational movement of the rod, in one direction or the other, along said axis of translation; and in that said first sealing means comprise at least one gasket mounted on said shaft. - a main body having a first duct for the inlet of the liquid to be filtered and a second duct for the outlet of the filtered liquid, said main body further comprising a hollow cylindrical portion; - a filter cartridge housed in said hollow cylindrical portion so as to divide a containment volume defined by said hollow cylindrical portion into a first volume, in fluid communication with said first duct, and a second volume, in fluid communication with said second duct; - valve means (116, 145) for controlling the flow of the liquid from said first duct to said first volume, said valve means (116, 145) comprising a valve seat interposed between said first duct and said first volume and a shutter member movable with respect to said valve seat between a closed position and an open position, in which it interrupts and allows, respectively, the flow of the liquid from said first duct to said first volume through said valve seat; - actuator means for controlling the movement of the shutter member between said closed position and said open position, wherein said actuator means comprise a rod, which is operatively associated with the shutter member and is shiftable along an axis of translation coinciding with its longitudinal axis, an actuator body, which is mounted on the main body and is configured to guide said rod in its translational movement of along said axis of translation, and a knob configured to be driven into rotation by a user to move said rod, and a shaft drivingly connected for rotation with said knob; and - first sealing means arranged inside the actuator body; in that said rod is locked with respect to rotation around said axis of translation and has, at a first end portion thereof facing away from the shutter member, a threaded cavity; in that the shaft comprises a threaded portion engaging in said threaded cavity of the rod, in such a way that the rotational movement of the knob, and therefore of the shaft, in one direction or the other, is converted into a translational movement of the rod, in one direction or the other, along said axis of translation; and in that said first sealing means comprise at least one gasket mounted on said shaft.