regnal
adjective
reg·nal ˈreg-nᵊl 
: of or relating to a king or his reign
specifically : calculated from a monarch's accession to the throne
in his eighth regnal year
Example Sentences
Recent Examples on the Web The same source reported that Charles considered making his regnal name George VII instead to honor his grandfather George VI. 
Andrea Vacchiano, Fox News, 9 Sep. 2022 Queen Elizabeth's eldest son, Charles, now becomes king, taking the regnal name of King Charles III. WSJ, 9 Sep. 2022 The question of William's regnal name is an interesting one, though. Caroline Hallemann, Town & Country, 8 Sep. 2022 In a performance rendered even lovelier by its contrast with the brashness of the previous five, Anna Uzele makes a touching creation of Catherine Parr, who probably did not in real life develop a theory of retroactive regnal sisterhood. New York Times, 3 Oct. 2021 When Vajiralongkorn was officially coronated in 2019, three years after his father’s death, the new king was given Rama X as his regnal name. Eamon Barrett, Fortune, 31 Oct. 2020
Andrea Vacchiano, Fox News, 9 Sep. 2022 Queen Elizabeth's eldest son, Charles, now becomes king, taking the regnal name of King Charles III. WSJ, 9 Sep. 2022 The question of William's regnal name is an interesting one, though. Caroline Hallemann, Town & Country, 8 Sep. 2022 In a performance rendered even lovelier by its contrast with the brashness of the previous five, Anna Uzele makes a touching creation of Catherine Parr, who probably did not in real life develop a theory of retroactive regnal sisterhood. New York Times, 3 Oct. 2021 When Vajiralongkorn was officially coronated in 2019, three years after his father’s death, the new king was given Rama X as his regnal name. Eamon Barrett, Fortune, 31 Oct. 2020 Word History
Etymology
Medieval Latin regnalis, from Latin regnum reign — more at reign
First Known Use
1612, in the meaning defined above