Sulfamonomethoxine (SMM) is widely used to inhibit Gram-positive and Gram-negative bacteria, and improper use of SMM is detrimental to human health and ecological stability. Therefore, a sensitive determination method is of great importance for monitoring SMM residues in water, meat, milk, eggs, etc. Herein, a Pt-functionalized S-doped graphitic carbon nitride (Pt/Sg- C3N4) was constructed for the electrochemical determination of SMM. The as-developed Pt3/ S3-g-C3N4 sensor showed a significant SMM determination performance. The electrochemical oxidation of SMM on Pt3/ S3-g-C3N4/GCE involves two electron transference and was limited by a diffusion process. The as-developed Pt3/ S3-g-C3N4/GCE sensor has good linearity in a wide range of 0.1–120 μmol/L and a remarkably low limit of detection (LOD) of 0.026 μmol/L for SMM determination. In addition, the sensor has high selectivity and anti-interference properties for SMM detection. Furthermore, this Pt3/ S3-g- C3N4/GCE sensor has good reproducibility and stability. Moreover, the recoveries were in the range of 89.6–112.2% for the detection of the SMM in a real sample of egg. The proposed Pt3/ S3-g-C3N4/GCE sensor shows great potential for practical applications in detecting trace amounts of antibiotics.