Thursday marked 44 years and 223 days on the throne for the Swedish king, breaking a national record dating back to the 1300s.
“After consulting the National Archives (Riksarkivet) it can be confirmed that as of today the king has sat on the throne longer than the 1300s king Magnus Eriksson,” Sweden's Royal Court wrote.
Magnus IV Eriksson was Sweden's king between 1319 and 1364, reigning for 44 years and 222 days, and was also king of Norway between 1319 and 1355.
The controversial medieval king ruffled feathers in his day by raising taxes to purchase the former Danish province of Skåne, facing a rebellion by his own son Eric XII, who ruled the southern parts of Sweden and Finland for a time while his father retained the rest.
Carl XVI Gustaf became king on September 15th, 1973, following the death of his grandfather Gustaf VI Adolf (Carl XVI Gustaf's father died in a plane crash in 1947).
Despite now certifying his unique place in Swedish history, the record king does not plan to mark it in any way, and is instead busy travelling home after a four-day state visit to Japan.
“The Royal Court will write about it on the website and social media, but the king won't do anything special to draw attention to it. He's working as normal,” the Swedish Royal Court's information secretary Irene Beertema Strömmer told news agency TT.
And though his longevity is impressive the Swedish king still trails some of Europe's other monarchs, including in neighbouring Denmark where Queen Margrethe II is in her 47th year on the throne, while the UK's Queen Elizabeth II has reigned for 66 years and counting.