The 2018–19 season was Celtic's 125th season of competitive football. They competed in the Scottish Premiership, League Cup, Scottish Cup, UEFA Champions League and UEFA Europa League. Celtic won all three domestic tournaments, completing an unprecedented treble treble.

A treble in association football is achieved when a club team wins three trophies in a single season. A continental treble involves winning the club's top-level domestic league competition, main domestic cup competition, and main continental trophy. Although winning a second-tier continental trophy has also been described as a continental treble, it is not as widely accepted. A domestic treble involves winning three national competitions—including the league title, the primary cup competition, and one secondary competition, such as a secondary cup or state-level league.