The 28-year-old arrives for his second spell at the club from Birmingham City, who recalled him from his loan at Kilmarnock at the beginning of the January window.
His current deal with the English Championship side expires at the end of this season.
Stewart had been in excellent form for Steve Clarke’s men, scoring eight goals in 16 appearances as he largely played the No.10 role in a team that’s managed to challenge for the Ladbrokes Premiership title this term.
The former Dundee attacker spent the 2017/18 campaign on loan at Aberdeen. Though he played 34 games he didn’t have as much of an impact, scoring only three times and later admitted fitness issues had played a part in his poor form.
He’ll now have a second chance under manager Derek McInnes, who’ll be looking for his side to maintain their challenge near the top of the table.
Aberdeen sit two points behind Kilmarnock and three behind each of Celtic and Rangers.