Abstract Background Morphine is an analgesic agent used for cancer pain management. There have been recent concerns that the immunosuppressant properties of morphine can also promote cancer metastasis. Morphine is an agonist for toll like receptor 4 (TLR4) that has a dual role in cancer development. The promotor or inhibitor role of morphine in cancer progression remains controversial. We investigated the effects of morphine on migration and metastasis of melanoma cells through TLR4 activation. Methods Mouse melanoma cells (B16F10) were treated with only morphine (0, 0.1, 1, and 10 μM) or in combination with a TLR4 inhibitor (morphine10 μM +CLI-095 1μM) for either 12 or 24 hours. Migration of cells was analyzed by transwell migration assays. Twenty C57BL/6 male mice were inoculated with B16F10 cells via the left ventricle of the heart and then randomly divided into two groups (n = 10 each) that received either morphine (10 mg.kg−1, sub-q) or PBS injection for 21 days (control group). Animals were euthanized and their lungs removed for evaluation of metastatic nodules. Results Morphine (0.1, 1, and 10 μM) increased cell migration after 12 hours (p < 0.001) and after 24 hours of treatment with morphine (10 μM) (p < 0.001). Treatment with CLI-095 suppressed migration compared to cells treated with morphine alone (p < 0.001). Metastatic nodules in the morphine-treated group (64 nodules) were significantly higher than in the control group (40 nodules) (p < 0.05). Conclusion Morphine increases the migration and metastasis of mouse melanoma cells by activating TLR4.