For purposes of the tax imposed by section 2001, the value of the taxable estate shall be determined by deducting from the value of the gross estate the amount of any estate, inheritance, legacy, or succession taxes actually paid to any State or the District of Columbia, in respect of any property included in the gross estate (not including any such taxes paid with respect to the estate of a person other than the decedent).
26 U.S. Code § 2058 - State death taxes
(a) Allowance of deduction
(b) Period of limitationsThe deduction allowed by this section shall include only such taxes as were actually paid and deduction therefor claimed before the later of—
a petition for redetermination of a deficiency has been filed with the Tax Court within the time prescribed in section 6213(a), the expiration of 60 days after the decision of the Tax Court becomes final,
an extension of time has been granted under section 6161 or 6166 for payment of the tax shown on the return, or of a deficiency, the date of the expiration of the period of the extension, or
(C) a claim for refund or credit of an overpayment of tax imposed by this chapter has been filed within the time prescribed in section 6511, the latest of the expiration of—
60 days from the date of mailing by certified mail or registered mail by the Secretary to the taxpayer of a notice of the disallowance of any part of such claim,
60 days after a decision by any court of competent jurisdiction becomes final with respect to a timely suit instituted upon such claim, or
Notwithstanding sections 6511 and 6512, refund based on the deduction may be made if the claim for refund is filed within the period provided in the preceding sentence. Any such refund shall be made without interest.