Inducible CRISPR-dCas9 transcription system has become a powerful tool for transcription regulation and sensing. Here, we develop a new concept of small-molecule-mediated split-aptamer assembly for inducible CRISPR-dCas9 transcription activation, allowing quantitative detection and imaging of S-adenosyl methionine (SAM) in live cells. This inducible transcription system is designed by integrating one fragment of a split SAM aptamer to guide RNA (gRNA) and the other to MS2 arrays. SAM-mediated reassembly of the split fragments recruits an MCP-fused transcription activator to the gRNA-dCas9 complex, activating the expression of a near-infrared fluorescent protein for imaging. We demonstrate that this inducible transcription system achieves quantitative detection of SAM with high sensitivity in live cells. Our system shows that methionine adenosyltransferase 1A (MAT1A) and MAT2A can both catalyze SAM production in live cells and the SAM levels in cancer cells can be increased via upregulation of MAT1A mRNA by epigenetic inhibitors. This split-aptamer assembly strategy could afford a new approach for controlling the CRISPR-dCas9 system, enabling conditional transcription regulation in response to endogenous metabolites in live cells.