The 31-year-old Serbian who was most recently at Paris St Germain but had his contract terminated by mutual consent towards the end of last year has also played for Chelsea, Atletico Madrid and PSV Eindhoven.
"Having played the majority of my career in Europe I wanted a new challenge in my life," Kezman told the club's website. "The opportunity to play for South China excites me and I feel I can offer the team many things. I have maintained fitness over the winter and feel ready to play and score goals.
"I want more success in my football career and believe South China FC can bring that, I have spoken to the manager and seen the team play and I believe this is the right club for me, I'm confident this will be a fantastic experience for all involved."
South China are Hong Kong's most successful club but are determined to claim continental glory this season in the AFC Cup and chairman Steven Lo believes the capture of Kezman and Butt shows the extent of the club's ambitions.
"To attract a striker of Mateja's quality is fantastic for South China, as a club we hope his ability and experience will be a great asset to our team," he said.
"Mateja has scored goals in many of the world's top leagues and we hope he will do the same for South China. Everyone at the club is overjoyed by the acquisition of Mateja and the fans should be delighted at what he can offer South China."