This study evaluates the cost-effectiveness of natural forest regeneration and plantations for climate mitigation. It estimates and maps the cost of greenhouse gas (GHG) abatement (US per tCO₂) for two common reforestation methods: natural regeneration and plantations. The analysis integrates new maps of implementation and opportunity costs, likely plantation genus, and carbon accumulation for both methods, accounting for carbon storage in harvested wood products. The results show that natural regeneration (46%) and plantations (54%) would each have lower abatement costs across about half the area considered suitable for reforestation in 138 low- and middle-income countries. Using the more cost-effective method at each location, the 30-year, time-discounted abatement potential of reforestation below US50 per tCO₂ is 31.4 GtCO₂ (24.2–34.3 GtCO₂ below US20–100 per tCO₂), which is 44% more than natural regeneration alone or 39% more than plantations alone. Reforestation offers 10.3 times more abatement below US20 per tCO₂ than the most recent IPCC estimate. Natural regeneration involves minimal human intervention, while plantations generally produce more wood products. Natural regeneration forests provide greater biodiversity, water provisioning, and erosion control. The cost-effectiveness of each method varies spatially, with natural regeneration being more cost-effective in regions such as Western Mexico, the Andean region, Southern Cone of South America, West and Central Africa, India, Southern China, Malaysia, and Indonesia, while plantations are more cost-effective in the Caribbean, Central America, Brazil, North, East, and Southern Africa, northern China, mainland Southeast Asia, and the Philippines. The study produces marginal abatement cost curves, which estimate how many tons of carbon dioxide could be removed by reforestation below any cost-per-ton. These curves can help determine the minimum carbon price incentive needed to induce land users to reforest. The results show that natural regeneration could remove up to 21.8 GtCO₂ below a cost of US50 per tCO₂, while plantations could remove up to 22.6 GtCO₂. Using the more cost-effective method at each site could remove 31.4 GtCO₂ below US50 per tCO₂. Latin America holds 56% of potential abatement from reforestation using the more cost-effective option below US50 per tCO₂, followed by Asia (33%) and Africa (11%). The top ten countries for potential cost-effective abatement are Brazil, China, Mexico, India, Myanmar, Colombia, Indonesia, Thailand, Democratic Republic of the Congo, and Venezuela. The study highlights the importance of considering spatial and temporal variations in abatement costs and the need for a globalThis study evaluates the cost-effectiveness of natural forest regeneration and plantations for climate mitigation. It estimates and maps the cost of greenhouse gas (GHG) abatement (US per tCO₂) for two common reforestation methods: natural regeneration and plantations. The analysis integrates new maps of implementation and opportunity costs, likely plantation genus, and carbon accumulation for both methods, accounting for carbon storage in harvested wood products. The results show that natural regeneration (46%) and plantations (54%) would each have lower abatement costs across about half the area considered suitable for reforestation in 138 low- and middle-income countries. Using the more cost-effective method at each location, the 30-year, time-discounted abatement potential of reforestation below US50 per tCO₂ is 31.4 GtCO₂ (24.2–34.3 GtCO₂ below US20–100 per tCO₂), which is 44% more than natural regeneration alone or 39% more than plantations alone. Reforestation offers 10.3 times more abatement below US20 per tCO₂ than the most recent IPCC estimate. Natural regeneration involves minimal human intervention, while plantations generally produce more wood products. Natural regeneration forests provide greater biodiversity, water provisioning, and erosion control. The cost-effectiveness of each method varies spatially, with natural regeneration being more cost-effective in regions such as Western Mexico, the Andean region, Southern Cone of South America, West and Central Africa, India, Southern China, Malaysia, and Indonesia, while plantations are more cost-effective in the Caribbean, Central America, Brazil, North, East, and Southern Africa, northern China, mainland Southeast Asia, and the Philippines. The study produces marginal abatement cost curves, which estimate how many tons of carbon dioxide could be removed by reforestation below any cost-per-ton. These curves can help determine the minimum carbon price incentive needed to induce land users to reforest. The results show that natural regeneration could remove up to 21.8 GtCO₂ below a cost of US50 per tCO₂, while plantations could remove up to 22.6 GtCO₂. Using the more cost-effective method at each site could remove 31.4 GtCO₂ below US50 per tCO₂. Latin America holds 56% of potential abatement from reforestation using the more cost-effective option below US50 per tCO₂, followed by Asia (33%) and Africa (11%). The top ten countries for potential cost-effective abatement are Brazil, China, Mexico, India, Myanmar, Colombia, Indonesia, Thailand, Democratic Republic of the Congo, and Venezuela. The study highlights the importance of considering spatial and temporal variations in abatement costs and the need for a global