Abstract: Against the background of intensifying global climate change, fully utilizing the efficient carbon sink function of seaweed cultivation is an important way to achieve China’s “double carbon” goal. To maximize the potential of seaweed cultivation as a carbon sink, it is essential to scientifically, accurately, and systematically calculate its carbon sequestration capacity. Most of the previous studies on the carbon sink of seaweed cultivation have overlooked the critical contributions of lost particulate organic carbon (POC) and released refractory dissolved organic carbon (RDOC) during the cultivation process. Based on the production data of seaweed cultivation from nine coastal provinces in China from 2000 to 2022 and considering the carbon sequestration formed by the release of POC and RDOC during seaweed cultivation, the long-term and short-term carbon sinks of seaweed cultivation in China were evaluated in this study. In addition, we have further analyzed the removal of nitrogen and phosphorus and the related economic benefits of the seaweed cultivation. The results showed that the average production of seaweed cultivation in the nine coastal provinces in China from 2000 to 2022 was 18.2×10⁵ t·a⁻¹, with an average cultivation area of 11.1×10⁴ hm²·a⁻¹. The average long-term carbon sink of seaweed cultivation in China was 2.70×10⁵ t(C)·a⁻¹, while the average short-term carbon sink was 4.75×10⁵ t(C)·a⁻¹. The average total carbon sink (long-term carbon sink + short-term carbon sink) was 7.45 × 10⁵ t(C)·a⁻¹, and the carbon sink of seaweed cultivation in China was expected to increase annually. Approximately, the total carbon sink of seaweed cultivation in China will reach 14.22×10⁵ t(C) by 2030. Fujian, Shandong, and Liaoning contributed approximately 91.5% of the total carbon sink, long-term carbon sink, and short-term carbon sink, with the greatest increase observed in the southern marine economic circle. The carbon sinks of seaweed cultivation in nine coastal provinces in China showed large differences owing to cultivation variety, aquaculture production, etc. Saccharina japonica had the highest carbon sequestration capacity among the seaweed species, and compared to other seaweeds, it required the smallest cultivation area to achieve China’s “carbon neutrality” goals. For long-term carbon sink, short-term carbon sink, and total carbon sink, the required cultivation area for Saccharina japonica to achieve China’s “carbon neutrality” goal was estimated to be 1.14×10⁷ hm², 0.65×10⁷ hm², and 0.41×10⁷ hm², respectively. According to the coastal seaweed production, the annual average removal of nitrogen and phosphorus by seaweed cultivation from 2000 to 2022 was estimated at 5.44×10⁴ t(N)·a⁻¹ and 0.66×10⁴ t(P)·a⁻¹, respectively. Furthermore, the release of oxygen was estimated at 1.99×10⁶ t(O₂)·a⁻¹, and the total environmental benefits of coastal seaweed cultivation in China reached 17.30 billion ¥ in 2022. The results of this study provide important insights into the development of seaweed cultivation in the context of achieving “carbon neutrality” and mitigating seawater deoxygenation and eutrophication.