Endometriosis is characterized by the presence of ectopic endometrial tissues. Mechanisms of tissue dissemination in endometriosis may be similar to those involved in tumor metastasis. We hypothesize that dysregulation of kisspeptin (KISS1), a metastasis suppressor in endometrial carcinoma, may contribute to the pathogenesis of endometriosis. In this study, we characterized the immunoreactivity of kisspeptin and its receptor, KISS1R, in eutopic and ectopic endometrial tissue of women with and without endometriosis, in proliferative and secretory menstrual cycle phases. Immunohistochemistry was performed using KISS1 and KISS1R antibodies on samples from women with (n = 35) and without (n = 14) endometriosis. Samples from women with endometriosis included eutopic endometrium (n = 20) samples, superficial endometriotic implants (SUP, n = 10) deep infiltrating endometriotic implants (DIE, n = 15), and ovarian endometriomas (OMA, n = 15). Immunoreactivity was quantified using histoscores. KISS1 and KISS1R immunoreactivity was significantly lower in eutopic endometrial stroma of women with versus without endometriosis, regardless of the menstrual cycle phase (P = 0.001 and P = 0.015 respectively). In endometriotic implants, KISS1 levels were significantly lower in both glandular and stromal components of DIE (P < 0.01) and OMA (P < 0.01) compared to SUP. KISS1R immunoreactivity was lower in the glandular component of OMA (P = 0.035) compared to SUP. KISS1 and KISS1R levels are lower in eutopic endometrial stroma from women with versus without endometriosis, consistent with a role for decreased KISS1 expression in the pathogenesis of endometriosis. As deeply invasive lesions showed lower KISS1 levels than superficial lesions, downregulation of KISS1 levels may contribute to implant invasiveness.