A method and system for dynamically watermarking media content based on transport-stream metadata, to facilitate action by a downstream entity. An intermediary in a media-distribution path could receive a transport stream carrying media content and carrying metadata that is not watermarked in the media content. The intermediary could then obtain from the transport stream the metadata that is not watermarked in the media content and could modify the media content at least by encoding into the media content a watermark that represents the obtained metadata. And the intermediary could then output at least the modified media content for transmission to a downstream entity. The downstream entity could thus obtain from the watermark in the media content the metadata and could use the metadata to facilitate carrying out one or more useful actions.