Chronic inflammation plays a key role in cancer pathogenesis. Aggressive thyroid cancer is associated with immune infiltration and systemic inflammation. Long pentraxin 3 (PTX3) is an inflammatory protein implicated in tumor progression. This study evaluates PTX3 plasma levels in patients with non-medullary thyroid cancer (TC) compared to benign thyroid disease and investigates its tissue expression. We prospectively included 55 TC patients: 42 papillary, 3 follicular, 4 oncocytic, 4 anaplastic (ATC), and 2 poorly differentiated (PDTC). The control group consisted of 32 patients with benign thyroid disease. PTX3 plasma concentrations were measured by ELISA, and tissue expression of PTX3 and CD68 was analyzed using immunohistochemistry. PTX3 plasma levels did not significantly differ between TC and controls, but patients with PDTC and ATC had markedly higher concentrations. Tissue analysis showed strong PTX3 expression in three of four ATC cases in tumor and stromal cells, whereas benign and differentiated thyroid tissues exhibited minimal staining. CD68 expression was positive in ATC, indicating tumor-associated macrophage infiltration, but a few cells were double-positive for PTX3 and CD68. Our findings suggest a possible association between PTX3 and aggressive TC, particularly ATC. Further studies are needed to validate these findings and elucidate the cellular origin and functional role of PTX3.