The 2010 World Cup-winner only joined Roma in 2020 and signed a three-year contract but was not part of new manager Jose Mourinho's plans -- having even been omitted from their preseason squad list.
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Pedro, who was given the No. 9 shirt at Lazio, will reunite with former boss Maurizio Sarri, having worked under him during their season together at Chelsea.
He made 40 appearances for Roma last season, scoring six goals in all competitions.
The 34-year-old is one of the most successful players in recent times, having won 20 major trophies at Barcelona, including five LaLiga titles and the Champions League three times, while he also won the Premier League, FA Cup and Europa League at Chelsea.
Pedro also enjoyed success at international level as he followed up the 2010 World Cup victory with a triumphant Euro 2012.
Roma and Lazio harbour one of Italy's fiercest rivalries and goalkeeper Astutillo Malgioglio was the last player to move directly between the clubs in 1985 when he made the same move as Pedro.