Dynorphin 1–17 (DYN 1–17) is biotransformed rapidly to a range of fragments in rodent inflamed tissue with dynorphin 3–14 (DYN 3–14) being the most stable and prevalent. DYN 1–17 has been shown previously to be involved in the regulation of inflammatory response following tissue injury, in which the biotransformation fragments of DYN 1–17 may possess similar features. This study investigated the effects of DYN 3–14 on lipopolysaccharide (LPS)-induced nuclear factor-kappaB/p65 (NF-κB/p65) nuclear translocation and the release of pro-inflammatory cytokines interleukin-1beta (IL-1β) and tumor necrosis factor-alpha (TNF-α) in differentiated THP-1 cells. Treatment with DYN 3–14 (10 nM) resulted in 35% inhibition of the LPS-induced nuclear translocation of NF-κB/p65. Furthermore, DYN 3–14 modulated both IL-1β and TNF-α release; inhibiting IL-1β and paradoxically augmenting TNF-α release in a concentration-independent manner. A number of opioids have been implicated in the modulation of the toll-like receptor 4 (TLR4), highlighting the complexity of their immunomodulatory effects. To determine whether DYN 3–14 modulates TLR4, HEK-Blue™−hTLR4 cells were stimulated with LPS in the presence of DYN 3–14. DYN 3–14 (10 μM) inhibited TLR4 activation in a concentration-dependent fashion by suppressing the LPS signals around 300-fold lower than LPS-RS, a potent TLR4 antagonist. These findings indicate that DYN 3–14 is a potential TLR4 antagonist that alters cellular signaling in response to LPS and cytokine release, implicating a role for biotransformed endogenous opioid peptides in immunomodulation.