The 30-year-old racked up 345 games for Liverpool after joining them from Brazilian side Gremio for just £5m in 2007.
His final game for Jurgen Klopp’s side came on the final day of last season as Liverpool beat relegated Middlesbrough 3-0.
Lazio see Lucas as a like-for-like replacement for Argentine midfielder and namesake Lucas Biglia, who has joined big-spending AC Milan.
The Brazilian international was a popular figure at Anfield, though he failed to really make himself an indispensible first team player. Following the departures of Xabi Alonso and Javier Mascherano (in 2009 and 2011 respectively), Lucas became more of a first team fixture, though a cruciate ligament injury sustained against Chelsea at Stamford Bridge in December 2011 ended what was arguably his best spell of football for the Reds.
Versatility was a key part of Lucas’ popularity, with the midfielder often filling in at centre-back. Last season he made 11 appearances as a centre-back and 14 in his preferred midfield position.
Lucas was named Liverpool’s Young Player of the Year in 2010 and Fans’ player of the year in 2011.
The former Gremio man was presented with a golden ’21’ (his squad number) on the final day of last season, to mark his 10 years at the club.