In enzymology, a 24-methylenesterol C-methyltransferase (EC 2.1.1.143) is an enzyme that catalyzes the chemical reaction

S-adenosyl-L-methionine + 24-methylenelophenol ${\displaystyle \rightleftharpoons }$ S-adenosyl-L-homocysteine + (Z)-24-ethylidenelophenol

Thus, the two substrates of this enzyme are S-adenosyl methionine and 24-Methylenelophenol, whereas its two products are S-adenosylhomocysteine and (Z)-24-ethylidenelophenol.

This enzyme belongs to the family of transferases, specifically those transferring one-carbon group methyltransferases. The systematic name of this enzyme class is S-adenosyl-L-methionine:24-methylenelophenol C-methyltransferase. Other names in common use include SMT2, and 24-methylenelophenol C-241-methyltransferase. This enzyme participates in the biosynthesis of steroids.

• Bouvier-Nave P, Husselstein T, Benveniste P (1998). "Two families of sterol methyltransferases are involved in the first and the second methylation steps of plant sterol biosynthesis". Eur. J. Biochem. 256 (1): 88–96. doi:10.1046/j.1432-1327.1998.2560088.x. PMID 9746350.