The roles of the serine/glycine metabolic pathway (SGP) have recently been evident in certain types of tumor physiology; however, the pathological relevance of SGP in undifferentiated thyroid cancer remains unexplored. Herein, we employed a comparative transcriptome analysis of bulk RNA sequencing coupled with single-cell RNA sequencing, showing that the high expression of SHMT2 and MTHFD2 is tightly associated with a low thyroid differentiation score (TDS) and poor clinical features in thyroid cancer. In addition, unique metabolic reprogramming of SGP-relevant pathways in dedifferentiated cancer cells, and a distinct trajectory of a subpopulation of tumor cells with a high mitochondrial one-carbon pathway was observed. More importantly, such dramatic changes are functionally relevant, as inhibition of SHMT2 significantly compromises mitochondrial respiration and decreases tumor size by upregulating TDS markers. Taken together, our results highlight the importance of the mitochondrial one-carbon pathway, including SGP, in undifferentiated thyroid cancer and suggest that SHMT2 is a novel therapeutic target.