The 27-year-old forward joined QPR from Lugano in 2024 after featuring for Slovenia at the European Championship in Germany that summer.
Celar made 24 appearances for the Hoops, scoring twice, including in a crucial victory over Cardiff City before a hamstring injury disrupted his campaign.
The striker spent last season on loan at 2. Bundesliga club Fortuna Dusseldorf, and has now returned to Switzerland to continue his career with Basel.
"Zan had a challenging start to English football, working hard to acclimatise to the league,” QPR CEO Christian Nourry told the club website.
"Last season, he was keen to have a platform in Germany to play his football but unfortunately that did not translate into minutes on the pitch.
"He moves to a renowned Swiss club and returns to a league where he finished up as top scorer 24 months ago."