The interiors of the Seimas are decorated with two stained glass pieces of art by Prof. Kazys Morkūnas, entitled Šventė (Feast) and Žalgirio mūšis (The Battle of Grunwald, 2011). The stained glass window Šventė (Feast, 1981), which currently decorates Building 1 of the Seimas, ranks among the largest stained glass windows in Lithuania and covers 180 square metres.

The artist devised a composition whereby he speaks to the audience through symbolic rather than realistic images of science, art, and motherhood.

Three decades later, the first part of the stained glass Žalgirio mūšis (The Battle of Grunwald, 2011) was unveiled in Building 2 of the Seimas to mark the 600th anniversary of this historic battle. The artwork, with the total area of 25 square metres, portrays Vytautas Magnus in a battlefield.