Exploring pathways for transitioning to clean energy to achieve climate change mitigation goals is essential. Addressing the economic impact of decarbonization options requires strategic planning to determine cost-effective CO₂ mitigation strategies. The assessment of marginal CO₂ abatement cost curves (MACC) is commonly used to link costs and emission reduction of decarbonization measures, with the cumulative CO₂ abatement potential and associated costs. This study aims to calculate a step-wise MACC to identify the most cost-effective order of investments in decarbonization measures for the German sector-coupled energy system from 2030 to 2045. It provides in-depth insights into the techno-economic, intertemporal, and intersectoral interactions of decarbonization options by applying a linear optimization model to determine the minimal cost of CO₂ abatement. The results show that the optimal strategy for decarbonization includes expanding wind and solar power capacity, and increasing demand-side flexibility, by integrating battery electric vehicles, power-to-heat, and power-to-gas. Results indicate that the interaction between the decarbonization measures is significant and that the integration of surplus electricity through sector-coupling can significantly reduce curtailment. Overall, the presented step-wise MACC approach can support policymakers in determining the most cost-efficient energy strategy by identifying the optimal technology mix to meet specific climate goals.