Abstract: A method and device for producing a y-fitting having a y-branch from a metal tube by hydroforming. The device includes a die defining a cavity having a cylindrical portion, in which the tube is placed, and a branch, opening into the cylindrical portion and having a shape corresponding to the y-branch; opposed pistons configured to move inside the die to exert thrust on each end of the tube; an injection mechanism configured to inject a fluid into the tube at high pressure so that metal flows into the branch; a counter-plunger arranged in the branch and in contact with that portion of the tube facing the branch; and a displacement mechanism configured to displace the counter-plunger between a first position parallel to the axis of the tube and a second position perpendicular to the axis of the branch.