Tempura originated in Japan in the 16th century, likely as a result of Portuguese Jesuit missionaries introducing deep-frying techniques to the country.
The exact origins of tempura are debated, but a popular theory is that Portuguese Jesuit missionaries introduced the Japanese to deep-frying techniques in Nagasaki.
The Portuguese word tempero means “seasoning”.
The Japanese adapted and refined the technique, creating tempura. The dish became a staple of Japanese cuisine during the Edo period (1603–1868).
Tempura’s popularity grew during the Edo period, and it became more accessible to the public when Hanaya Yohei opened the first tempura-dedicated restaurant in Tokyo.
As tempura became more popular, regional variations began to develop. For example, in the Kansai region, a style of tempura known as “tenkasu” or tempura flakes developed.
Tempura has spread beyond Japan, with Thai cuisine embracing its own unique version of the dish.
This is a terrific site for learning how to make perfect Japanese tempura.
Very detailed with great inside tips.
https://sudachirecipes.com/authentic-tempura-batter/
My Japanese cookbook suggests the Quatuor Tempora or Ember Days observances, in which abstinence from meat is observed, were picked up by the Japanese from the visitors.