The Singapore Computer Emergency Response Team (SingCert) issued an alert on Wednesday to hosting providers of more than 80 websites as the sites were found to be defaced.
Three of the Reform Party's websites were among these defaced sites.
The hosting providers were notified so that they could look into the hacking and take precautions.
No government sites were among those affected, SingCert said in a statement late Wednesday night.
"Cyber security is everyone's responsibility. Businesses and individuals are urged to take the necessary precautions to enhance their cyber security," said SingCert, a unit of the Infocomm Development Authority.
Businesses and individuals can visit the GoSafe Online website at www.gosafeonline.sg to learn more about how to protect themselves against cyber threats or seek assistance.