Cordycepin, or 3'-deoxyadenosine, is a derivative of the nucleoside adenosine, differing from the latter by the absence of oxygen in the 3' position of its ribose part. It was initially extracted from fungi of genus Cordyceps, but is now produced synthetically.

Because cordycepin is similar to adenosine, some enzymes cannot discriminate between the two. Therefore, it can participate in certain biochemical reactions (for example, be incorporated into an RNA molecule, thus causing the premature termination of its synthesis)