The Greek international becomes the club's tenth recruit of the window.
With Odsonne Edouard expected to depart, Ange Postecoglou has bolstered his attack with last season’s Eredivisie top scorer.
Giakoumakis netted 26 goals for VVV Venlo despite the club’s relegation to the second tier.
The 26-year-old has signed a five-year deal.
He said: “Celtic is one of the world’s great football clubs with the best fans around. It is a club I have admired for so long and it will now be an honour to wear this famous jersey. To know that the manager wanted me so much to join Celtic is really humbling and it makes me know I have made the right choice.
“We have some exciting challenges this season in Scotland and in Europe and I will be doing all I can to help the team as much as possible and bring our supporters success.”