<p>The polyunsaturated fatty acid docosahexaenoic acid (DHA) has impressive anti-inflammatory and pro-resolution properties but its therapeutic use in PCa requires pre-clinical evidence. Here, the transgenic adenocarcinoma of the mouse prostate (TRAMP) was used as a pre-clinical model to assess the effect of DHA intake on the inflammatory microenvironment and proliferative and survival pathways in early- and late-stage disease. TRAMP mice were fed with standard rodent or DHA-enriched diet (<i>DHA-d</i>) for 4 (early stage) or 10 weeks (late stage). The ventral prostate was evaluated using histopathological, immunohistochemical, and western blotting analysis. Serum samples were collected for TNF-α measurement. Histopathological analysis showed that <i>DHA-d</i> delayed the progression of PCa and the development of in situ and well-differentiated carcinoma at both ages. Dietary DHA reduced cell proliferation and increased apoptosis by inhibiting the Akt pathway in late-stage disease and activating ERK1/2 signaling in early-stage disease. <i>DHA-d</i> down-regulated pyroptosis and up-regulated necroptosis in the late stage. The intake of DHA reduced CD4⁺ T-cell and M2-like macrophage and increased CD8⁺ T-cell infiltration only in the late stage. TNF-α systemic level was down-regulated by <i>DHA-d</i> in both periods but the TNF R1 protein level in the prostate diminished only in the late stage. Overall, <i>DHA-d</i> has a protective effect on prostate carcinogenesis of TRAMP mice by stimulating a low inflammatory and anti-tumor feature, reducing the CD4⁺/CD8⁺ ratio, and downregulating the M2-like macrophage profile. Such immunomodulatory effects suggest a protective action of dietary DHA in the early stages of PCa.</p>