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The teenager was confronted by the suspects at traffic lights near the junction with Windsor Place, on Stuttgarter Strasse in Cardiff city centre, at around 9pm on Friday, March 6.
The passenger on the moped is reported to have grabbed the e-bike before riding away, with the stolen bike last seen heading towards Newport Road.
Both suspects are described as white and were wearing balaclavas.
Anyone who witnessed the robbery or has information is asked to contact South Wales Police quoting reference 2600071286, via the online form, by calling 101, or by contacting Crimestoppers anonymously on 0800 555 111.
