Sydney Sweeney and Fiancé Jonathan Davino Separated in January

Sydney Sweeney and Jonathan Davino have been living separate lives for a few months now ... TMZ has learned.

A source familiar with the situation tells TMZ ... Sydney and Jonathan are not together romantically and haven't been since January, when they separated.

The couple were last photographed together Jan. 20 and we broke the story in early February that they had postponed their May wedding.

More signs of trouble in paradise soon emerged ... as we first told you, Sydney checked in to the Beverly Hills Hotel for an extended stay without Jonathan ... she hung around the property with her girlfriends and then took them with her to Paris Fashion Week.

All the while, Jonathan was in Los Angeles ... helping with the construction going on at Sydney's mansion ... but she was nowhere in sight.

Sydney's also deleted a photo of her kissing Jonathan from her Instagram ... which may have been the kiss of death for their relationship.

SS and JD got engaged way back in 2022 after sparking dating rumors in 2018 ... but now it's tough to envision them ever walking down the aisle.