Enzymes involved in RNA capping of SARS-CoV-2 are essential for the stability of viral RNA, translation of mRNAs, and virus evasion from innate immunity, making them attractive targets for antiviral agents. In this work, we focused on the design and synthesis of nucleoside-derived inhibitors against the SARS-CoV-2 nsp14 (<i>N</i>7-guanine)-methyltransferase (<i>N</i>7-MTase) that catalyzes the transfer of the methyl group from the <i>S</i>-adenosyl-l-methionine (SAM) cofactor to the <i>N</i>7-guanosine cap. Seven compounds out of 39 SAM analogues showed remarkable double-digit nanomolar inhibitory activity against the <i>N</i>7-MTase nsp14. Molecular docking supported the structure-activity relationships of these inhibitors and a bisubstrate-based mechanism of action. The three most potent inhibitors significantly stabilized nsp14 (Δ<i>T</i><sub>m</sub> ≈ 11 °C), and the best inhibitor demonstrated high selectivity for nsp14 over human RNA <i>N</i>7-MTase.