In this paper, we present a distributed interference-aware medium access control (DI-MAC) for IEEE 802.15.7 visible light communications (VLCs), which improves the spatial efficiency of VLC devices deployed in specific service areas. The DI-MAC adaptively adjusts the backoff exponent (BE) of each VLC device by considering the number of adjacent VLC devices causing the interference, which are called VLC interferers. Therefore, it reduces interference between the VLC devices and unnecessary waiting time for channel access. To this end, the DI-MAC conducts VLC interferer discovery by using the IDs of VLC interferers obtained via hello messages from adjacent VLC devices and then performing a BE adaptation to determine the BE of each VLC device based on the number of VLC interferers. An experimental simulation was conducted to evaluate the performance of the DI-MAC, and the results show that the DI-MAC obtains a higher aggregate throughput performance than the existing IEEE 802.15.7 VLC.