We report the use of graphitic carbon nitride, g-­C3N4 (g­-CN) based electrochemical genosensor for the ultrahigh sensitive detection of DNA, taking breast cancer (BRCA1) gene as a model gene. The sensor uses the traditional sandwich detection strategy, in which the capture probe DNA (DNA­-c) was immobilized on g-­CN modified glassy carbon electrode (GCE) surface. Use of a gold nanoparticle cluster (AuNPC) as electrochemical label in reporter probe DNA and g-­CN as immobilization platform for capture probe DNA in the sensor design leads to highly sensitive electrochemical detection of BRCA1 gene. The sensor could detect down to 10 attomolar target DNA. Endonuclease reaction provided high selectivity to the mismatched DNA and the sensor was found to be stable and reproducible.