This disclosure describes a video co-watching system that can efficiently, flexibly, and securely provide access to, and synchronize, co-watching digital video content across multiple participant devices while providing live digital video chat streams between the participant devices. For example, the video co-watching system can enable the participant devices to simultaneously present co-watching digital video content together with a presentation of a video chat. In addition, the video co-watching system can synchronize the co-watching digital video content at each participant device while also facilitating video chatting between the participant devices by transmitting a playback modifier signal to cause one participant device to perform a modification to the playback made at another participant device.