Katzav has served a quarter of his seven-year sentence, which entitles him to leave confinement for a day under Israel's prison rehabilitation policies, the Israeli broadcaster Arutz Sheva reported.
Katzav resigned as president in July 2007 after allegations of sexual assault and sexual harassment surfaced. He was convicted in December 2010 of two counts of rape of former employees.
The ex-president had been convicted "of acts that didn't take place," his brother, Lior, told journalists outside Maasiyahu Prison.
"The battle to overturn his conviction continues, the truth will come to light, we haven't heard the last word yet," Lior Katsav said.
Moshe Katsav refused to answer questions from journalists as he left the prison.