The Brazilian defender returns to the club where he spent the second half of the 2015/16 season.
Bernardo signed for Albion from RB Leipzig on a four-year deal in July 2018 for a reported £9m.
Head coach Graham Potter said, “Bernardo is at a stage in his career where he wants to be playing regular football.
“This move should give him the opportunity to do that. We wish him well for the rest of the season and we will watch his progress with interest.”
Bernardo made his 50th Albion appearance in the 1-0 defeat to Manchester City last week – his seventh appearance of the campaign.
He also netted his first goal in an Albion shirt earlier this season – a header against Portsmouth in the Carabao Cup second round.
The move comes just days after Brighton recalled their summer signing Michal Karbownik from his loan with Polish top-flight club Legia Warsaw.
The 19-year-old left sided player joined Albion on a four-year deal on transfer deadline day in October from Warsaw, heading straight-back out on loan to the Polish capital.
Head coach Graham Potter said, “We have made the decision to bring Michael into the club and into the squad.
“He has been a regular for Legia Warsaw, and last season was a key player for them as they won their domestic league.
“He has a lot of adaptability; he can play either side of the defence, as well as in midfield. He’s an exciting young player who we are looking forward to having in the group.”
Karbownik made his Ekstraklasa debut for Legia in August 2019, going onto make 28 appearances that season as they lifted the Polish top-flight title.