Rocard, who served as prime minister from 1988 to 1991 under President François Mitterrand, made his controversial comments during an appearance on news show Le Grand Journal on Canal+.
"This man quite clearly has a mental illness," Rocard said of Strauss-Kahn, who has recently had sexual aggression charges against him dropped in New York.
This made it "difficult to control his impulses," said Rocard. "It's a shame, as he had real talent."
The ex-prime minister was also invited to comment on the current line-up of candidates who are seeking to win the Socialist party nomination to be the presidential candidate in next year's elections.
He noted that "the most popular" is "certainly François Hollande."
Hollande is the current front-runner in the race, with recent polls putting him ten points ahead of his closest rival, former party leader and mayor of Lille, Martine Aubry.
"The best placed technically, because she has been general secretary of the party, is Martine Aubry," said Rocard. "We will see…they are both people of high quality."
Rocard was less complimentary about another contender, Ségolène Royal, when asked whether he would be able to work with her as well.
"I hope not because I don't believe she has the capacity for the role and I don't think she will be elected," he said.
When reminded that Royal, who was the presidential candidate for the Socialists in 2007, believes she could win, he replied crisply, "we live in a society of free speech; the right to say what you like is a fundamental human right."