Abstract: 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), provide