Isometrus thwaitesi is a species of scorpion in the family Buthidae endemic to Sri Lanka.

Pedipalps and legs are yellowish, with spots.

Fingers and manus of pedipalps are yellowish, and spotted.

Flagellum laminiform, short, and basally broadened.

Median lobe of hemispermatophore is narrow, and distally truncate, with strong, curved dorsal carina.