"The fight against corruption is no excuse for flagrant due process violations and preventing people from mounting an adequate defense," Michael Page, HRW's deputy Middle East director, said in a statement on Tuesday.
"Given their track record of abuse, the Saudi authorities should make fundamental reforms to the justice system to ensure that the accused will not be railroaded in unfair legal proceedings."
Military and judicial officials were among the 298 people arrested over allegations of bribery and embezzlement amounting to 379 million Saudi riyals ($101m), Saudi Arabia’s so-called anti-corruption watchdog Nazaha said.
Nazaha said the arrests came after it investigated 674 state employees, but it neither named any of the suspects nor stated when its inquiry took place.
It was the latest government crackdown on what officials describe as endemic corruption in the kingdom.
A campaign against corruption launched in 2017 saw hundreds of elite princes, ministers and businessmen held at the luxury Ritz-Carlton hotel in the capital Riyadh.
Many were held for weeks, but most were released after agreeing to significant financial settlements. Authorities said they recovered more than 400 billion Saudi riyals ($107bn).
The anti-corruption sweep led by Crown Prince Mohammed bin Salman has been labeled by many critics as a shakedown and a power grab.
The latest surge of arrests coincides with a royal purge this month that saw the detention of Prince Ahmed bin Abdulaziz al-Saud, a brother of King Salman, and the monarch's nephew Prince Mohammed bin Nayef, who was previously crown prince.
Sources said they were arrested for allegedly plotting a palace coup to unseat the crown prince, heir to the Saudi throne.
But another source close to the Saudi royal court said the detentions were a "messaging exercise" to crush any opposition to the crown prince before his succession to the throne.