What was the first pokemon ash caught in Sinnoh?

Answer 1

Ash entered the Sinnoh region with Pikachu and Aipom, Team Rocket arrived and took Pikachu away, but their hot air balloon went down and Pikachu flew into the middle of a forest, which is where Dawn met Pikachu and tried to catch him. Ash was determined to find Pikachu , and he found a Starly and caught it so it could search from the sky.

So practically, Starly was his first in Sinnoh region.

Answer 2

Ash had brought Pikachu with him to the Sinnnoh region and his female Aipom had stowed away on the boat that was going from the Kanto region to the Sinnoh region however the first official Pokémon capture that Ash Ketchum made while in the Sinnoh region was by catching Starly which eventually later evolves to Staravia and then finally to Staraptor.