A polypurine (guanine)/polypyrimidine (cytosine)-rich sequence within the proximal promoter region of the human RET oncogene has been shown to be essential for RET basal transcription. Specifically, the G-rich strand within this region consists of five consecutive runs of guanines, which is consistent with the general motif capable of forming intramolecular G-quadruplexes. Here we demonstrate that, in the presence of 100 mM K+, this G-rich strand has the ability to adopt two intramolecular G-quadruplex structures in vitro. Moreover, comparative circular dichroism (CD) and DMS footprinting studies have revealed that the 3'-G-quadruplex structure is a parallel-type intramolecular structure containing three G-tetrads. The G-quadruplex-interactive agents TMPyP4 and telomestatin further stabilize this G-quadruplex structure. In addition, we demonstrate that the complementary C-rich strand forms an i-motif structure in vitro, as shown by CD spectroscopy and chemical footprinting. This 19-mer duplex sequence is predicted to form stable intramolecular G-quadruplex and i-motif species having minimum symmetrical loop sizes of 1:3:1 and 2:3:2, respectively. Together, our results indicate that stable G-quadruplex and i-motif structures can form within the proximal promoter region of the human RET oncogene, suggesting that these secondary structures play an important role in transcriptional regulation of this gene.