How to connect to an Amazon PostgreSQL database using SSL
Many connection options aren't available as command line flags. That's one of them. Use a connection string, e.g.:

psql "host=XXXX.rds.XXXX.com sslmode=require sslrootcert=rds-ssl-ca-cert.pem port=XXXX user=XXXXX dbname=XXXX"


(Syntax corrected)