By train:
Take bus no. 175 or night bus no. 611 from the airport to Warszawa Centralna station.
Several trains a day depart from the station to Poznań Główny station, including those operated by carriers like Tanie Linie Kolejowe, InterCity, EuroCity, as well as express and fast trains. The journey takes about 2 hours. All routes are handled by companies belonging to the PKP group.

By bus:
You can also get to Poznań by a PKS bus, via a few connections, from Warszawa Zachodnia or Warszawa Stadion bus terminals, or by using a direct bus from the airport bus terminals.