London 2012: Giovanni Cernogoraz wins trap shoot-off
The 29-year-old beat Italy's Massimo Fabbrizi into silver with six hits in the decider after the pair totalled 146 in qualifying and the final.
Great Britain's Ed Ling scored 118 in qualifying and did not make the final.
Cernogoraz came into the final as an underdog after qualifying in sixth place but the European champion missed just one shot to force a shoot-out with world number two Fabbrizi.
Diamond had earlier looked on course for his third trap gold medal at the Games after equalling the Olympic record in qualifying with a perfect 125 hits at the Royal Artillery Barracks.
However, the 40-year-old Australian could not keep up the pace and missed five shots in the final on his way to finishing fourth.
The gold medal was Croatia's second at the Games following Sandra Perkovic's victory in the women's discus.