An insert (400) for a rainwater downpipe (300) is fluidly coupled between a first section (310) and a second section (320) of the rainwater downpipe (300). A rainwater reservoir (500) is defined within the insert (400) to store the rainwater received from the first section (310) therein. The rainwater reservoir (500) includes a side surface (510), a bottom surface (520) and an outlet opening (522) fluidly coupled to the second section (320). A valve element (530) is disposed within the rainwater reservoir (500) to allow or deny the passage of the stored rainwater in the rainwater reservoir (500) through the outlet opening (522). The insert (400) is characterized in that the side surface (510) of the rainwater reservoir (500) further defines a rainwater harvesting opening (549) adapted to allow access to the rainwater stored in the rainwater reservoir (500).