A drain valve (10) implantable surgically in the eye (O) of a patient for the treatment of glaucoma, comprising a main body (11), and a drainage tube (12), connected at one end with the main body (11) and provided to be implanted in the eye (O) so as to penetrate with its distal tip inside the anterior chamber (CA) of the eye globe (GO), so as to allow the drainage outwards of the aqueous humour (UA), wherein the valve (10) is characterized by a series of significant improvements, including: 1) an extractable configuration (12a, 12b) of the drainage tube (12), to allow the adaptation (f1) of the length of the drainage tube (12) during the surgical operation to implant the valve (10); 2) a flattened or flat shape in section (12′) of the drainage tube (12, 12a, 12b) suitable for reducing the dimensions in the radial direction (IR) of the drainage tube (12) with respect to the surface of the eye globe (GO); 3) a connection free from encumbrances between the tip portion (12f) of the drainage tube (12), provided to penetrate inside the eye globe (GO, CA), and the remaining part (12b′) of the drainage tube (12); 4) a series of retaining bags or compartments (11b), formed along the lower or soffit surface (S″) of the main body (11) in contact with the surface of the eye globe (GO), wherein these retaining bags (11b) are associated with respective through holes (11a) extended through the same main body (11) of the valve and are suitable for receiving and retaining the aqueous humour (UA), so as to improve lubrication of the eyeball in the zone of the valve, once implanted; 5) a divergent fan-shaped opening (11c) formed on the extrados (S′), not in contact with the eye globe, of the main body (11) of the valve, in order to convey the aqueous humour (UA) and improve the lubrication around the drain valve (10), once implanted; and 6. a modified configuration, with respect to conventional drain valves, such as to allow, during the surgical operation, an arrangement, closer to the iris, of the holes (11d) for the insertion of the yarn for fixing the valve to the surface of the eye globe. Thanks to these improvements the drain valve (10) ensures considerably improved and superior performances with respect to the currently known drain valves in use in the medical field for treating glaucoma.