An adjustable and sealable jet nebulizer for bottles able to be elastically deformed by squeezing is mounted on the mouth of the neck (2) of a bottle (3). A conduit for air (16) and a conduit for liquid (17) are obtained coaxially in a cylindrical body (4) of the nebulizer, which is engaged with a screw-on cap (28), provided with a central exhaust orifice (29). The screw-on cap (28) has a cylindrical wall (30), to be inserted between the conduit for air (16) and the conduit for liquid (17), to create a mixing chamber (33) with variable geometry. The conduit for liquid (17) has on its upper end an arm (26) oriented upwards, bearing, at its free end, a tip cap (27), able to be inserted into the central exhaust orifice (29) whilst the screw-on cap (28) is screwed onto the cylindrical body (9), until sealing the nebulizer.