The invention relates to a 3D flexible pouch to be filled with a biopharmaceutical product, produced by assembling two wall elements and two gussets. At least one connection port can be provided for filling and/or emptying. A substantially parallelepipedic configuration is obtained in a filled state by unfolding the gussets and folding the flaps of the two wall elements. A welded seam is produced in a join portion formed at one end and cut out in a V shape, so as to directly connect the two wall elements. In a substantially flat configuration of the empty flexible pouch, the join portion projects axially outwards in relation to the gussets and is defined by two edges which are oblique in relation to a longitudinal axis of said pouch.