The bookmaker said the acquisition is aimed at strengthening its position in the Nordic region, where per capita spending on gambling is among the highest in Europe.
Ladbrokes will pay an initial 36 million sterling to acquire Sponsio, with an additional 4 million sterling due if future growth targets are achieved.
It said the acquisition will be earnings enhancing in the first year of ownership.
Ladbrokes signed a partnership agreement with Sponsio in 2001 to expand its business in the Nordic countries, with the region now the biggest earner for Ladbrokes eGaming outside of the UK.
Following the acquisition, Douglas Roos, the managing director of Sponsio, will head up Ladbrokes Scandinavian division.
A spokesman for Ladbrokes said the company would be “ramping up” its presence in the Nordic markets after the deal.
He said the company aims to build betting shops across the region, but this could be a “few years down the track”.