A unitized seal for use in a gas spring seal assembly. The unitized seal includes a rigid thermoplastic insert having an elastomeric sealing layer surrounding the insert and bonded to the insert. The insert provides rigidity to the seal and includes a central, axial opening that receives and guides a rod slidably engaged with the insert. The sealing layer engages the interior surface of the gas spring cylinder to prevent the pressurized gas contained within the gas spring from escaping around the seal. The sealing layer also includes a central, axial opening aligned with the passage in the insert that slidably engages the rod and further prevent gas from escaping the gas spring cylinder. The unitized seal may be formed in a single or two-step process. In the single step process, the insert and sealing layer are formed by injection molding processes in a single mold and are bonded to one another by a subsequent covulcanization process. In the two-step process, the insert is formed in a first injection molding step, and has an adhesion promoter applied to the exterior of the insert. The coated insert is then placed within a second mold that is used to form the sealing layer about the insert in a subsequent injection molding process. The adhesion promoter intimately bonds the sealing layer to the insert forming the unitized seal.