We have developed a novel label-free oligonucleotide sensor that possesses β-cyclodextrin (β- CD) as a hybridization-amenable signal repressor and ferrocene (Fc) as a signal generator with two different redox potentials for single-stranded and double-stranded states. The probe gives us an electrochemical ratiometric response based on the redox currents at these two potentials, revealing an oligonucleotide recognition event. The sensor is based on an artificial oligonucleotide probe, β-CD-DNA-SH, and FcUT immobilized on gold electrode surfaces. Electrochemical measurements showed two redox peaks of Fc redox reactions, with the higher potential peak corresponding to the redox reaction of Fc forming the inclusion complex with the terminal β-CD, and the lower one corresponding to the redox reaction of Fc not forming the inclusion complex. The ratios of these two peak currents were 0.815 and 0.398 for the target DNA and mismatched DNA, respectively. A sequence-specific electrochemical ratiometric response was observed. The sensors prepared from the developed probe are anticipated to detect oligonucleotides even when a deterioration in effectiveness might be expected due to rough treatment such as repeated use or unsuitable storage conditions. To the best of our knowledge, this is the first report of a ratiometric electrochemical oligonucleotide sensor based on one signal-generating moiety with two redox potentials.