The G-quadruplex structure in the genome is an important drug target because it regulates gene expression and the genome structure. Several small molecules that bind the G-quadruplex have been developed, but few artificial G-quadruplex binding proteins have been reported. We previously reported a novel G-quadruplex DNA binding protein (RGGF) engineered using the Arg-Gly-Gly repeat (RGG) domain of TLS (translocated in liposarcoma). Here we show that RGGF recognizes DNA loops in the G-quadruplex and preferentially binds G-quadruplex DNA with long loops. Furthermore, RGGF binds to G-quadruplex DNA of the c-myc promoter in vitro and represses c-myc transcription in vivo. On the basis of these findings, G-quadruplex binding protein engineered from the RGG domain will be useful for investigating G-quadruplex transcriptional function in the genome.