Background <p>Anaplastic thyroid cancer (ATC) is one of the most aggressive and lethal malignancies. ATC tumor cells exhibit marked overexpression of collagen family genes; however, the mechanisms underlying tumor cell-driven collagen remodeling remain poorly understood. Tumor-associated macrophages (TAMs) constitute a substantial proportion of the ATC tumor microenvironment, yet whether and how TAMs influence collagen remodeling in ATC tumor cells remains unclear.</p> Methods <p>We combined single-cell RNA sequencing and bulk transcriptomic data from public databases to explore the role of TAMs in collagen remodeling of tumor cells in ATC. Additionally, in vivo mouse tumor models were employed to assess tumor growth, followed by immunofluorescence analysis of tumor tissues.</p> Results <p>Single-cell transcriptomic analyses revealed pronounced myeloid enrichment and a dominant mesenchymal-like malignant epithelial state in ATC. This population exhibited a collagen-high transcriptional program associated with increased collagen deposition and unfavorable outcomes in the broader thyroid carcinoma context. CIBERSORTx-based deconvolution of multiple bulk RNA-seq cohorts independently confirmed expansion of myeloid cells in ATC and a positive correlation between tumor and myeloid proportions. Further characterization identified an ATC-enriched <i>SPP1</i>⁺ TAMs subset with strong extracellular matrix remodeling signatures. Cell-cell communication and spatial inference analyses highlighted SPP1-CD44 signaling as a predicted interaction between <i>SPP1</i>⁺ TAMs and mesenchymal ATC cells. In vivo inhibition of SPP1 partially restrained tumor growth and collagen accumulation. Pseudotime analysis further suggested <i>VSIG4</i>⁺ TAMs as an upstream state associated with <i>SPP1</i>⁺ TAM emergence.</p> Conclusions <p>Targeting the SPP1 axis may disrupt macrophage-driven collagen remodeling and offer new therapeutic opportunities for ATC.</p> Clinical Trial Number <p>Not applicable.</p>