Minimum S3 policy required for awscli s3 sync to work.

iam_policy.json

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": "s3:ListObjects",
            "Resource": "*"
        },
        {
            "Sid": "VisualEditor1",
            "Effect": "Allow",
            "Action": [
                "s3:PutObject",
                "s3:GetObject",
                "s3:PutObjectAcl"
            ],
            "Resource": "arn:aws:s3:::bucket-name/*"
        },
        {
            "Sid": "VisualEditor2",
            "Effect": "Allow",
            "Action": "s3:ListBucket",
            "Resource": "arn:aws:s3:::bucket-name"
        }
    ]
}

Here is what I use in 2023. The statements are generated with help from policy_sentry, but equally you chould use AWS's IAM Access Analyzer.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "S3ReadBucket",
            "Effect": "Allow",
            "Action": [
                "s3:GetBucketLocation"
            ],
            "Resource": [
                "arn:aws:s3:::bucket-name"
            ]
        },
        {
            "Sid": "S3ReadObject",
            "Effect": "Allow",
            "Action": [
                "s3:GetObject"
            ],
            "Resource": [
                "arn:aws:s3:::bucket-name/*"
            ]
        },
        {
            "Sid": "S3WriteObject",
            "Effect": "Allow",
            "Action": [
                "s3:AbortMultipartUpload",
                "s3:DeleteObject",
                "s3:PutObject"
            ],
            "Resource": [
                "arn:aws:s3:::bucket-name/*"
            ]
        },
        {
            "Sid": "S3ListBucket",
            "Effect": "Allow",
            "Action": [
                "s3:ListBucket",
                "s3:ListBucketMultipartUploads"
            ],
            "Resource": [
                "arn:aws:s3:::bucket-name"
            ]
        },
        {
            "Sid": "S3ListObject",
            "Effect": "Allow",
            "Action": [
                "s3:ListMultipartUploadParts"
            ],
            "Resource": [
                "arn:aws:s3:::bucket-name/*"
            ]
        }
    ]
}