The airline says it will offer passenger flights every Thursday and Sunday from March 24th - and the price will include a free one way train ticket from Southend to London Liverpool Street station.
More information is on the Sea Air website, although Southend Airport is yet to confirm the news.
Osijek is the fourth largest city in Croatia, and the largest economic and cultural centre in the eastern region of Slavonia. It is located on the right bank of the river Drava.