Gold clusters have been electrodeposited on a boron-doped diamond (BDD) electrode by scanning the potential from 0.7 V to 0.0 V (vs. 3 M KCl-Ag/AgCl reference) in a solution of 0.5 mM KAuCl4 and 1.0 M KCl. The cluster-modified diamond (Au/BDD) electrode has been used to investigate the oxidative properties of dopamine (DA) and ascorbate (AA). The modified diamond electrode shows a higher activity for DA oxidation than AA; the oxidation potential of DA shifted to a less-positive potential (0.11 V) than that of AA, which oxidized at 0.26 V, and DA possesses a much higher peak current than that of AA. The reversibility of the electrode reaction with DA is significantly improved at the Au/BDD electrode, which results in a large increase in the square-wave voltammetric peak current, with a detection limit of 0.1 μM in the presence of a large excess of AA. The Au/BDD electrode shows excellent sensitivity and good selectivity for DA detection. A self-assembled monolayer (SAM) of mercaptoacetic acid on the Au clusters was used to provide an antifouling effect as the negative CO2– groups repulse negative ascorbate and attract positive dopamine in pH 7.4 buffer. After pre-absorption, the SAM/Au/BDD electrode could detect 1.0 nM DA in a linear range from 10 nM to 10 μM in the presence of 10–4 M AA.