A method of joining two portions of an underwater pipeline for conducting fluid and/or gas on the bed of a body of water, the method including the steps of: joining a telescopic sleeve, having a first coupling head, to a first end of a first pipeline portion above the body of water; positioning the first end, the telescopic sleeve, and the first coupling head in a first given position on the bed of the body of water; joining a second coupling head to a second end of a second pipeline portion above the body of water; positioning the second end and the second coupling head in a second given position, close to the first coupling head, below the body of water; and connecting the first and second coupling head hermetically below the body of water.