CGTN reported the man, on a holiday in Pathum Thani, said he was shocked to see the reptile staring back at him out of his toilet.
In a TikTok video going viral across social media, at first, only the slithering tongue of the reptile can be seen from inside the pot.
The man, later identified by British newspaper Metro as Jason Kingman from Kent, said it was his girlfriend Chantima Chairisuk, who first noticed something inside the pot.
She said she has seen Asian water monitor lizards, which have a mildly venomous bite, living in their garden but it was the first time she encountered one in her toilet.