The patriarch died of a heart attack on Monday, aged 92. His death came just hours after a prostate operation.
He had been head of the Church since 1986. In 2000 he asked for forgiveness for concessions the Church had made during communist rule.
The leaders of other Orthodox churches are expected to join many mourners at the funeral.
Patriarch Teoctist was widely praised for hosting the late Pope John Paul II in Romania in 1999 - the pontiff's first visit to an Orthodox country in modern times.
About 87% of Romanians are Orthodox Christians, according to census figures.