The 31 year-old, making the seventh defense of his green belt, was cut in the opening round on his left eyelid but managed to overcome challenger Omari Kimweri on points.
Kimweri, 34, was harshly deducted a point for what was an accidental head clash but already faced an uphill battle against the more talented champion.
In the end, Menayothin came out a convincing winner on the cards as judges scored in favor of the home fighter 117-110 and 118-109 twice.
Now 47-0, Menayothin is just two wins away from equalling Floyd Mayweather and Rocky Marciano’s 49-0 record, although the Thai’s record has been considerably padded with one-sided non-title wins over the years.
In the co-feature, Petch Sor Chitpattana (or CP Freshmart, not to be confused with Knockout CP Freshmart) made it to 41-0 with a fourth round knockout of a debutant competitor.
Down under in Australia, the Moloney brothers continued their identical runs as both moved to 13-0 with respective stoppages.
Bantamweight Andrew halted Aramis Solis in the third session, whilst super-bantamweight Jason took two rounds more to dispatch of Emanuel Armendariz at Melbourne Park Function Centre in Victoria.