Berri, 80, is head of the Shi'ite Amal Movement and has been a close ally of the heavily armed Shi'ite Hezbollah since the end of the 1975-90 civil war. Outgoing Prime Minister Saad Hariri, a Hezbollah opponent, has declared his support for him.
Berri is running unopposed for the post reserved for a Shi'ite in Lebanon's sectarian power-sharing system. In anticipation of Berri's re-election, his office issued a statement urging supporters to avoid celebratory gunfire.
Another Hezbollah ally, Elie Ferzli, is a leading candidate to be elected as deputy parliament speaker, reflecting a shift in the political landscape in favor of Hezbollah since the last Lebanese general election in 2009.
Ferzli, like Berri and Hezbollah, has close ties to the Syrian government of President Bashar Assad.
Parties and individuals who back Hezbollah's possession of arms won at least 70 of parliament's 128 seats. The last time Lebanon held an election, in 2009, an anti-Hezbollah alliance led by Hariri and backed by Saudi Arabia won a majority.
The deputy speaker position, reserved for a Greek Orthodox Christian, has been held by a Hezbollah opponent since 2005, the year Syrian troops were forced to withdraw from Lebanon after the assassination of Rafik Hariri, Saad's father.
Hariri has declared his opposition to Ferzli, who is closely associated with the era of Syrian domination of Lebanon.
He is one of a group of pro-Hezbollah and pro-Damascus politicians whom the May 6 election returned to public office for the first time since 2005. Ferzli was information minister at the time of the Hariri assassination.
Once parliament has elected its speaker, President Michel Aoun, a Maronite Christian, will consult MPs on their choice for the post of prime minister, which is reserved for a Sunni.
Hariri is widely expected to be named premier again and faces tough negotiations to form a coalition government that will group all Lebanon's main parties, including Hezbollah.
Hariri lost more than a third of his seats, many of them to Hezbollah or Hezbollah-allied politicians. Hariri has blamed a new electoral system and shortcomings by his Future Movement for the losses. He has launched an internal shake-up of the party.
The make-up of the new parliament has raised expectations the incoming government will move to widen ties with Syria, particularly as Aoun and other Lebanese leaders seek the return to Syria of some one million refugees in Lebanon.
The staunchly anti-Hezbollah Lebanese Forces (LF) party, led by Maronite politician Samir Geagea, has said its MPs will cast blank ballots in Wednesday's vote. The LF almost doubled its number of MPs seats in the election, winning 15 seats.
The LF has nominated its own candidate for the position of deputy speaker, Anis Nassar.