The 2021 season in the city of Kyoto peaked on 26 March, according to data collected by Osaka University.
Increasingly early flowerings in recent decades are likely to be as a result of climate change, scientists say.
The records from Kyoto go back to 812 AD in imperial court documents and diaries.
The city has experienced an unusually warm spring this year.
The previous record there was set in 1409, when the season reached its peak on 27 March.
The blossoms, "sakura" in Japanese, last only for a few days, but their appearance is tremendously important, both economically and culturally. Friends and family get together, and Instagram is awash with pictures.
The dataset for cherry blossom season in Japan is especially valuable because it goes back so far.
"In Kyoto, records of the timing of celebrations of cherry blossom festivals going back to the 9th Century reconstruct the past climate and demonstrate the local increase in temperature associated with global warming and urbanisation," according to an earlier paper published in the scientific journal Biological Conservation.