As the parties took their places in the courtroom yesterday before the judges arrived, Mr Ruto appeared relaxed, laughing and smiling with his lawyers. Joshua arap Sang, his co-accused, gave a reporter the thumbs-up sign.
Mr Ruto and Mr Sang are charged with co-orchestrating a post-election bloodbath five years ago, working with co-conspirators to murder, deport and persecute supporters of rival political parties in Kenya's Rift Valley region.
"The crimes of which Mr Ruto and Mr Sang are charged were not just random and spontaneous acts of brutality," said Fatou Bensouda, the ICC's prosecutor, describing the charges in court.
"This was a carefully planned and executed plan of violence. Mr Ruto's ultimate goal was to seize political power for himself and his party in the event he could not do so via the ballot box."
It is the first time such a senior official has appeared in court to face international justice while still serving in office. Kenya's President Uhuru Kenyatta, Mr Ruto's former rival and now his political ally, will also face trial on similar charges of crimes against humanity, beginning in November.
The cases have split public opinion, and witness testimonies of the violence in 2007-08 that killed more than 1000 people could stir tension.
The cases are also a major test for prosecutors at the decade-old Hague-based ICC, who have had a low success rate and face accusations of focusing on African countries.