Carnosic acid (CA) and carnosol (CS) are two structurally related diterpenes present in rosemary herb (Rosmarinus officinalis). Although several studies have demonstrated that both diterpenes can scavenge free radicals and interfere in cellular processes such as cell proliferation, they may not necessarily exert the same effects at the molecular level. In this work, a shotgun proteomics study based on stable isotope dimethyl labeling (DML) and nano-liquid chromatography-tandem mass spectrometry (nano-LC-MS/MS) has been performed to identify the relative changes in proteins and to gain some light on the specific molecular targets and mechanisms of action of CA and CS in HT-29 colon cancer cells. Protein profiles revealed that CA and CS induce different Nrf2-mediated response. Furthermore, examination of our data revealed that each diterpene affects protein homeostasis by different mechanisms. CA treatment induces the expression of proteins involved in the unfolded protein response in a concentration dependent manner reflecting ER stress, whereas CS directly inhibits chymotrypsin-like activity of the 20S proteasome. In conclusion, the unbiased proteomics-wide method applied in the present study has demonstrated to be a powerful tool to reveal differences on the mechanisms of action of two related bioactive compounds in the same biological model.