Matvejevic was given a five-month prison sentence by the Zagreb Municipal Court on November 2 for libeling and offending writer Mile Pesorda in an article entitled "Our Taliban" which appeared in Jutarnji List daily four years ago.
Sanader said in a statement that he could not comment on the sentence given to Matvejevic because as prime minister he could not interfere in courts' decisions.
He added, however, that as a member of the Croatian Writers' Association and the Croatian PEN, he felt it was unacceptable to hand down such sentences against writers today.
He also said that democracy entailed responsibility for what one said or wrote and that no one could be freed from this responsibility.
Sanader said he joined in the public debate on the Matvejevic sentence because everyone should invest more effort in attempts at mutual understanding and tolerance.