Celtic has bolstered its quad in two key positions with the deadline day arrivals of Greece international Giakoumakis and emerging Portuguese midfield star Joao Filipe Jota.
Wantaway French forward Odsonne Eduard, meanwhile, sealed a move to Premier League club Crystal Palace.
The news will come as a relief to Celtic fans still smarting from the 1-0 loss to fierce rival Rangers in the weekend's Old Firm derby - a match in which a flat-looking Eduard missed a golden chance to put his side into the lead.
Giakoumakis topped the Eredivisie scoring charts last season with 26 goals from 30 matches, the 26 year-old set to be the panacea to Celtic's lack of depth in the forward line.
The Greece star is set to slot straight into the Bhoys' starting side.
Joining Giakoumakis in Glascow is rising Portuguese star Jota, who has represented his country at every group. The fleet-footed winger - who spent last season on loan at then-LaLiga club Real Valladolid - can play on the right or left and appears suited to the attacking football espoused by Australian manager Ange Psotecoglou.
“It’s unbelievable, I’m very happy to join such a historical club as Celtic and I just can’t wait to get to Paradise and show what I can do," Jota said. ‘Hopefully I can give joy to the fans and show what I can do.’
“I’ll try to play football to score goals, to make assists and to put smiles on the faces of all of the Celtic fans. I will be very motivated to do that. It’s a very important club and I’m so happy to join it."
On a busy deadline day of activity for the Hoops, Scott Robertson headed to Crewe Alexandria on loan; Leigh Griffiths joined Dundee FC on loan; Lee O'Conner moved to Tranmere Rovers on loan and Ryan Christie sealed a permanent move to Bournemouth.