The move was allegedly aimed at destabilising a rival centre-left government in power at the time.
Berlusconi lost his parliamentary immunity last year when he was expelled from the Senate over a tax fraud conviction.
He is not due at the hearing as he does not have to attend under Italian law.
The billionaire - who denies the charges - is accused of giving 3m euros (Â£2.5m; $4m) in 2006 to Sergio De Gregorio, then a senator from the anti-corruption Italy of Values party, to switch to Berlusconi's People of Freedom party and thus undermine the government of the time.
Profile: Silvio Berlusconi
Q&A: Silvio Berlusconi on trial
A former journalist, Valter Lavitola, is also on trial for allegedly acting as intermediary for the bribe.
The trial is being held in Naples as it was the seat occupied by Mr De Gregorio.
The first two days are expected to be largely procedural.
The trial is the third ongoing case against Italy's former prime minister, who is appealing against a prison sentence for having sex with an under-age prostitute and abuse of office, and another for leaking a confidential police wiretap.
He was also found guilty of tax fraud last year - his first definitive conviction.
Berlusconi remains an influential political force, sealing a deal last month with his centre-left rivals in the governing Democratic Party (PD).
Under the agreement, his Forza Italia party, the largest opposition bloc, will back electoral and constitutional changes.
Former Italian PM Silvio Berlusconi has gone on trial in Naples for allegedly bribing a senator to join his party in 2006, in the latest case against him.