I have a spring boot project where we load SSM values as properties, one of these values is a JSON formatted string, so I want to know if there is a possibility to tell SSM to read the value from an S3 bucket so that I can save a x.json file instead of a formatted string.
No, SSM Parameter Store does not have a native integration with S3 to allow you to pull a file as the parameter value.
What you can do is create a parameter with the S3 URL (the bucket and object key) and then have your code read the prop from the file after first fetching the config file location from SSM Parameter Store.
But the parameter value is just a string, and since JSON is also just a string, it means you can put whatever string you want--formatted or not--as the parameter value, as per one of the comments. As long as the string can be parsed as valid JSON you can use it as such.