Fans expressed delight that the character became the strongest Pokemon trainer in the entire world more than 25 year after its first episode aired.
A tweet from the official Pokemon twitter account congratulating Ash received nearly 200,000 likes.
The series started as a videogame and has been on the air for over 1,200 seasons.
Since 1997 when Ash Ketchum met Pikachu, an electric mouse-like creature, the anime has followed him in his quest for Pokemon Masterhood.
Over the last 25-years, viewers have followed his journey to becoming the strongest Pokemon trainer. The series sees Pokemon trainers battle against pocket monsters to catch mythical creatures.
In addition to the animated TV series, the original Japanese Game Boy game inspired a trading game, over 20 movies, and a variety of merchandise including pogs (collectible cardboard disks) and soft toys.
Many seasons of the anime follow Ash as he travels through different fictional regions to catch Pokemon. His goal is to earn eight gym badges that will qualify him for the Pokemon League.
However, the latest series, Pokemon Ultimate Journeys, worked in a different way. Ash has traveled around the world instead of being restricted only to one region.
Viewers have already seen Ash win two tournaments, the Orange League from the Orange Islands Season and the Alola League of Pokemon: Sun or Moon.
Ash needed to overcome strong trainers throughout his travels in order to win the World Championship.
Four episodes were required to see Ash’s final fight against Leon, the former champion of the world. It also featured the return of some former anime companions who were retired from the series, such as Dawn, Misty, Brock and Misty.
Sarah Natochenny (voice actor for Ash) tweeted: “YESSSS!!! Our boy is the greatest !!!!” While a few adult fans of the show tweeted that they were “emotional” after the finale.
It is unknown if Ash or Pikachu will make a comeback for the anime’s next series.
It is a tradition that a new season in anime coincides with the release date of new Pokemon video game. Scarlet and Violet, the two newest Pokemon video games, will be released Friday 18 November.