How do you recover you RDS master user username?

The master username is now listed in the RDS Dashboard.

1. Select the "Instances" listing and expand the DB instance.
2. Press the looking glass tab on the left.
3. The master username is listed in the left column.

Hope this helps!

The master user name can be recovered with the rds-describe-db-instances command.

If you are using aws cli-2, command shall become :

aws rds describe-db-instances --region ap-south-1

It is 2019 now. It is 'Databases' on the sidebar, 'Configuration' on the navigation bar. And look for username.

With the new updated RDS UI, the username is present in the Details section of the Instance Information screen.

1. From the List of your running Instances, click an instance name to see information about the instance.
2. See the "Details" section of the information screen.

scroll down in that screen to see the "Details" section.

Here is the command that you can use to get the master username of your RDS:

aws --profile=prod rds describe-db-instances
    --region us-east-1
    --db-instance-identifier YOUR-RDS-IDENTIFIER-HERE
    --query 'DBInstances[*].MasterUsername'

Note: Replace the prod with your aws profile and YOUR-RDS-IDENTIFIER-HERE with your rds instance identifier

It will give you the master username like this:

[
    "root"
]

Hope that might help future reader.

I do not know how to directly find the aws admin username in 2019 AWS, however, you can see it in a snapshot. Just follow:

Main RDS console > Snapshots > A snapshot of your database > Details > Master username

Using AWS CLI to get all Instance names + the respective Master users:

aws rds describe-db-instances | grep -e \"DBInstanceId -e Master

SELECT grantee 
FROM  sys.dba_role_privs 
WHERE granted_role='RDS_MASTER_ROLE' 
AND admin_option ='NO';

TL;DR

Select the database and click top-right on modify

Here is the description how to do it:

The third field is the password.