Is the untaken portion of a deceased persons RMD subject to his estate or does it belong to the spouse as his beneficiary?

If the spouse files a joint return for the year of death, it can be included n the 1040 form. It could also be attributed to the estate.

What is a RMD? Sounds like a type of property interest, but more information is needed.

If it identifies her as the beneficiary, she takes the remainder. Otherwise, it belongs to the estate and distribution made by the terms of the will.

The remaining amount of RMD at the time of the decedent's death is considered IRD, income in respect to a decedent, and is taxable to the estate or the recipient. Who gets the money is a different question. The documents (retirement account, beneficiary designation, trust, if there is one, or any other documents) need to be reviewed to determine where the money goes. If the spouse was the beneficiary then the final joint return of the decedent and spouse could include the RMD balance.

Generally, the plan/policy language is going to govern and that will dictate whether it passes through the estate or directly to a beneficiary. Please review the plan language to see if it specifies a beneficiary explicitly.