In organic chemistry, a methylidyne group or just methylidyne is a neutral part of a molecule (a substituent or functional group) with formula , consisting of a carbon atom bonded to a hydrogen atom by one single bond and to the rest of the molecule by one triple bond.  For example, a methylidyne group is present in n-methylidyne-1-hexanaminium, .
The name "methylidyne" is also used for the methylidyne radical (carbyne) , the same two atoms not bound to any other atom.