Yes - a song that is released as a single and then as an album should always maintain the same ISRC. This is because an ISRC uniquely identifies the recording to which it was assigned, so any changes not pertaining to the actual sound recording itself should not affect a recording's ISRC code. In short, the recording that is released as a single and then via an album are exactly the same - so each release does not qualify as its own unique recording, thus does not require separate ISRCs.
However, if you release a work, and then go on to release various versions of the sound recording, such as a karaoke version or remix, each version does require a new ISRC since the changes made are related to the sound recording itself, and the other versions now each represent a unique recording that differs in sound from the original release. For a full, more detailed description of situations that require a new ISRC to be assigned, we recommend consulting the most recent ISRC Handbook, which is available here .
If you are releasing through a distributor like CD Baby or Distrokid, they will automatically assign the same ISRC to both releases, so you won't need to worry about this! However, if you are distributing your music independently, you should make sure you are not creating a new ISRC for the album version of a single (assuming the recordings being released are identical).
Interested in learning more about ISRCs? Check out our article covering everything you should know about them here !