Finnair’s code will be added to airberlin flights from Oslo, Gothenburg, Stockholm and Copenhagen to Berlin, and from Copenhagen to Düsseldorf.
The carrier is also adding its code to airberlin-operated daily flights from Düsseldorf and Berlin to Abu Dhabi.
Airberlin, meanwhile, will add its code to Finnair flights from Helsinki to Tallinn, Turku, Tampere and Oulu.
The new codeshares will be available from October 28.
Finnair vice president, Paavo Virkkunen, said: “We are very pleased with the growing cooperation with airberlin. This deepening of our codeshare arrangements helps us enhance our route network in the Nordic countries and build feeder traffic through our Helsinki hub.
Paul Gregorowitsch, airberlin’s chief commercial officer, said: “Right from the beginning, the codeshare cooperation with our oneworld partner Finnair is developing very well for both of us, especially the routes between Germany and Helsinki.
“We are happy to offer our flight guests the first-time opportunity to book a domestic flight in Finland as well as to Estonia and thus, to book flights within an extended Nordic route network.”
Finnair and airberlin have been sharing code on flights between Finland and Germany and to points onward in Italy and Austria since 2010.
In March this year, airberlin joined Finnair in the oneworld alliance.
Finnair carries more than eight million passengers a year and connects 11 cities in Asia with more than 50 destinations in Europe.
Airberlin has a fleet of 152 aircraft and flies to 170 destinations in 40 countries.