"The Chamber vacates the trial date of May 28, 2013," the three-judge bench said in a decision made at the court's headquarters in The Hague. "The new start date for trial... will be rendered in due course," they added.
Ruto, 46, faces three counts of crimes against humanity for his role in deadly violence that erupted in Kenya after the last elections in 2007.
Prosecutors last month asked to add five witnesses to their list, prompting Ruto's lawyers to ask for a so-called status conference to discuss the case's progress.
Having heard these submissions, "the Chamber considers that it is not feasible, at this stage, to retain May 28 as the start of the trial," the judges said.
ICC judges have already postponed Ruto's trial once before, saying they considered it important that he has enough time to properly prepare his case.
Ruto will go on trial with his co-accused, radio boss Joshua arap Sang.
Some 1,100 people died in bloodshed after the last elections over allegations of vote rigging, shattering Kenya's image as a beacon of regional stability.
Both Ruto and Kenyatta deny the charges.
