The ruthenium complex sodium trans-[tetrachloridobis(1H-indazole)ruthenate(iii)] (KP1339/IT-139) showed preclinical activity in a variety of in vivo tumor models including a highly predictive colon cancer model. The compound has entered clinical trials, where patients experienced disease stabilization accompanied by mild side effects. KP1339, a GRP78 inhibitor, disrupts endoplasmic reticulum (ER) homeostasis leading to cell death. The PERK/eIF2α-branch of the ER plays an essential role in the cascade of events triggering immunogenic cell death (ICD). ICD makes dying cancer cells 'visible' to the immune system, initiating a prolonged immune response against the tumor. As some metal-based chemotherapeutics such as oxaliplatin are able to induce ICD, we investigate whether KP1339 could also trigger induction of the ICD signature. For this, we employ a three-dimensional colon cancer spheroid model and show for the first time that the treatment with KP1339, a ruthenium-based complex, triggers an ICD signature hallmarked by phosphorylation of PERK and eIF2α, exposure of calreticulin on the cell membrane, release of high mobility group box 1 and secretion of ATP.