The cis/trans geometry of olefins is known to dramatically influence the thermal and mechanical properties of polyalkenamers. Yet, polymerization methods that allow the practitioner to efficiently control this parameter are scarce. Herein, we report the development of a stereoretentive acyclic diene metathesis (ADMET) polymerization that capitalizes on the unique reactivity of dithiolate Ru carbenes combined with cis monomers. These Ru catalysts exhibit exquisite retention of the cis geometry and tolerate many polar functional groups, enabling the synthesis of all-cis polyesters, polycarbonates, polyethers, and polysulfites. Additionally, the stereoretentive ADMET is characterized by low catalyst loadings and tolerance toward trans impurities in the monomer batch, which should lend to ready implementation at large-scale. Remarkably, modulation of the reaction conditions including temperature and reaction time leads to an erosion of stereoretention, thereby permitting a stereocontrolled synthesis of polyalkenamers with predictable cis:trans ratios. The impact of the cis:trans alkene content within the polymer backbone on the thermal properties was clearly demonstrated through differential scanning calorimetry and thermogravimetric analysis. Stereocontrolled ADMET provides a unique synthetic approach toward materials with precise structures and tailored properties.