Adult S. balanoides grow up to 15 millimetres (0.6 in) in diameter, and are sessile, living attached to rocks and other solid substrates.

[2] When the tide rises to cover the barnacles, the operculum opens, and feathery cirri (modified thoracic appendages)[4] are extended into the water to filter food from the seawater.

Its distribution is limited in the north by the extent of the pack-ice and in the south by increasing temperature which prevents maturation of gametes.

[3] Although capable of living in the sublittoral zone, S. balanoides tends to be restricted to the intertidal by predation and by competition from species such as the blue mussel, Mytilus edulis, and the algae Ascophyllum nodosum and Chondrus crispus.

[7] Semibalanus balanoides is a filter feeder, using its thoracic appendages, or cirri, to capture zooplankton and detritus from the water.

Plankton levels are highest in Spring and Autumn, and drop significantly during Winter, when the barnacles are dependent on reserves of food which they have stored.

[2] Predators of Semibalanus balanoides include the whelk Nucella lapillus, the shanny Lipophrys pholis,[2] the sea star Asterias vulgaris,[7] and nudibranchs.

The isopod Hemioniscus balani occurs from France to the Faroe Islands and the Oslofjord, and from Labrador to Massachusetts, and is a parasite of S. balanoides, effectively castrating the barnacle if it is heavily infested.