The 21-year-old midfielder joined from Standard Liege for an "undisclosed fee on a long-term contract, subject to a work permit being confirmed" to become Michael Beale's second signing after Todd Cantwell.
Celtic manager Ange Postecoglou earlier ruled out any deadline-day signings and Oliver Abildgaard proved to be the only departure before midnight with the Danish midfielder joining Hellas Verona following nine substitute appearances for the Hoops.
Giorgos Giakoumakis was still officially a Celtic player at midnight but he is expected to join Major League Soccer side Atlanta United, whose transfer window opened on Tuesday and runs until April 24.
Postecoglou said on Tuesday afternoon: "Anyone who is playing football and has been a meaningful contributor won't be leaving in this window.
"Potentially the ones who leave are ones who haven't had a lot of regular game time. There are four or five in the squad, I am not going to talk about individuals."
Reports claimed Celtic rebuffed loan offers for Stephen Welsh from Sheffield Wednesday and Dundee United.
It was a busy day of comings and goings at Pittodrie. Aberdeen signed 22-year-old Ajax goalkeeper Jay Gorter and 21-year-old Blackburn forward Dilan Markanday on loan and 30-year-old former Hull and Swindon defender Angus MacDonald on a deal until the end of the season.
Skipper Anthony Stewart left for MK Dons on loan and Vicente Besuijen returned to the Netherlands to join Excelsior Rotterdam on loan. The Eredivisie side have a purchase option for the summer.
St Mirren signed 18-year-old Southampton wing-back Thierry Small and brought in Tony Watt on a loan deal from Dundee United after selling two players. Ethan Erhahon's move to Lincoln was confirmed and 17-year-old playmaker Dylan Reid travelled to Crystal Palace for a medical.
Kilmarnock were looking to add after the surprise exit of Kyle Lafferty and Oli Shaw's move to Barnsley for an undisclosed fee but there were no announcements by the deadline.
Hibernian signed 21-year-old United States international striker Matthew Hoppe on loan from Middlesbrough and manager Lee Johnson hinted there could be activity in the free-agent market.
St Johnstone landed Dundee striker Zak Rudden on loan but there were no more announcements after Callum Davidson stated they would be busy up until midnight.
Ross County signed striker Simon Murray from Queen's Park following three near misses elsewhere while Kazeem Olaigbe left Dingwall following a loan spell from Southampton.
Hearts failed with a fresh bid for Sheffield Wednesday's Callum Paterson on a quiet deadline day in Gorgie.
There were two in and two out at Motherwell with more on the way into Fir Park.
The Steelmen sold defender Sondre Solholm to Odds BK for an undisclosed fee and Connor Shields was loaned to Queen's Park. They left it late to announce the arrival of 22-year-old former Celtic attacker Jack Aitchison from Barnsley on a free transfer and 20-year-old Brighton left-back James Furlong on loan.
Chief executive Alan Burrows stated "we've got a bit more to do" with announcements expected on Wednesday.
Dundee United failed to get a forward in to replace Watt before the deadline but signed 19-year-old Huddersfield defender Loick Ayina on loan.
United got significant news from elsewhere late in the day when Harry Souttar's £15million move from Stoke to Leicester went through. United could receive a sell-on fee of up to £3million, more than six years after selling the defender to the Potters.