RDS have only one super user by default when the instance is created. But we can create users the same permissions as the master user.
This article explains the steps to create additional super user in RDS PostgreSQL.
Step1: Login to the RDS PostgreSQL instance as super user.
#psql -h <host> -U <username> -p <port>
Here we need to replace host, username and password with the required values.
Step2: Create the new user.
>CREATE ROLE name WITH PASSWORD 'password' CREATEDB CREATEROLE LOGIN;
Here we need to replace name, password with the required values.
Step3: Assign super user privileges to the new user.
>GRANT rds_superuser TO name;
Where replace name with the with new username that created.