Peritoneal metastases of high-grade serous ovarian cancer (HGSOC) are small-sized deposits with superficial growth toward the peritoneal cavity. It is unknown whether integrity of the peritoneal elastic lamina (PEL) correlates with the peritoneal tumor microenvironment (pTME) and whether neoadjuvant chemotherapy (NACT) affects the pTME. We explored integrity of PEL, composition of pTME, effects of NACT, and the prognostic implications in patients with extensive peritoneal metastases of HGSOC. Peritoneal samples (n = 69) were collected during cytoreductive surgery between 2003 and 2016. Clinical data were collected from medical charts. Integrity of PEL was evaluated with elastic stains. T cell (CD3, CD8) and M2-macrophage markers (CD163) were scored using algorithms created in definiens tissue studio. Patients with a disrupted PEL (n = 39; 57%), more often had residual disease after surgery (p = 0.050), compared to intact PEL. An intact PEL was associated with increased intraepithelial (ie) CD8+ cells (p = 0.032), but was not correlated with improved survival. After NACT, increased ieCD3+ cells were shown, compared to no-NACT (p = 0.044). Abundance of total CD3+ and CD8+ cells were associated with PFS (multivariate HR 0.40; 95%CI 0.23–0.69 and HR 0.49; 95%CI 0.29–0.83) and OS (HR 0.33; 95%CI 0.18–0.62 and HR 0.36; 95%CI 0.20–0.64). M2-macrophage infiltration was not correlated with survival. NACT increases abundance of ieCD3+ cells in peritoneal metastases of HGSOC. Increase of CD3+ and CD8+ cells is associated with improved PFS and OS. This suggests that CD3+ and CD8+ cells may function as prognostic biomarkers. Their role as predictive biomarker for chemotherapy or immunotherapy response in HGSOC warrants further research.