Adu-Adjei had been involved in Andoni Iraola’s first team over pre-season, scoring once in the Premier League Summer Series against Everton, after a prolific campaign last season.
But the young striker has now left the club after four years and looks set to play first team football at Rijeka, the team currently sat sixth in the top flight.
He may also get a taste of European football, with Rijeka currently leading PAOK 1-0 after the first leg of their Europa League play-off, with the second leg on Thursday.
Adu-Adjei joined Bournemouth in 2021 after spells with the Brentford and Fulham youth teams, joining the club’s development squad.
He has had a few loan spells while at the Cherries, first going to Poole Town in 2023, before spells at Leyton Orient and Carlisle United in the Football League.
His most recent loan was at Carlisle for the first half of last season, where he scored two goals across 17 appearances.
But he was recalled in January as Bournemouth struggled for injuries, appearing in four Premier League match day squads as an unused substitute.
Adu-Adjei starred in Alan Connell’s development squad, though, with 25 goals in 16 games helping to fire the Cherries to the Professional Development League play-off final.
The 20-year-old posted a farewell message on social media, saying: “After 4 amazing years [at AFC Bournemouth] it’s time for the next chapter.
“Forever grateful. Thank you to all my teammates, staff and fans for the support.”