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The diet of Dermasterias imbricata varies with locality. On the exposed coast, it eats primarily sea anemones and some compound sea squirts, and in sheltered waters, mostly sea cucumbers as well as encrusting sponges and sea pens. Even though the sea cucumber Psolus chitonoides contains toxic saponins, 39 per cent of D. imbricata surveyed were feeding on it. All those observed in Gabriola Passage were feeding on bottom detritus. This sea star evokes a swimming-escape response in the sea anemones Stomphia didemon and S. coccinea, and presumably would eat them if it was able to catch them. Stomphia species detect imbricatine, an alkaloid chemical produced by D. imbricata. Seven other species of sea stars cause Stomphia species to release but do not eat them. D. imbricata also stimulates juveniles of the anemone Urticina piscivora to release their attachment; large U. piscivora may defend themselves by attacking the sea star, though they later regurgitate it. In California, D. imbricata usually feeds on the anemone Corynactis californica. In an aquarium, it prefers the sea anemones Anthopleura or Metridium; but in the ocean, these anemones are not regular prey items because in the intertidal zone they live too high and in the subtidal zone they grow too large for the sea star.

D. imbricata breeds from April to August. It sheds gametes via three paired gonopores in each arm radius. The yellow-to-orange eggs are 200 micrometres in diameter. In the Vancouver Island region, juveniles successfully settled and survived just once in five years. The scale worm Arctonoe vittata is found on D. imbricata; a study at Bodega Bay discovered that the sea star was attracted to the scale worm more than to food items, suggesting that the relationship may be of mutual benefit. The parasitic barnacle Dendrogaster can infect D. imbricata's reproductive organs.