Ricin toxin chain A (RTA), a byproduct of the production of castor oil from castor bean plants, can cause serious health problems if ingested, inhaled, or injected. In this project, we have designed and tested an electrochemical biosensor that is capable of ‐‐ and sensitive enough ‐‐ to detect small, biomedically relevant concentrations of RTA. To create this biosensor, we drew from an existing aptamer, previously shown to bind to RTA, and rationally designed a custom biosensor scaffold that coupled RTA binding to a conformational change. Ultimately, this oligonucleotide biosensor design allows voltammetric interrogation to detect RTA concentration in complex media (such as coffee, blood, and river water). This biosensor is convenient and collects real‐time data, offering beneficial applications in monitoring of areas of castor oil production, and furthermore offers potential diagnostic applications in assessing Ricin exposure.