Background/Objectives: Angiogenesis in glioblastoma (GBM) is a multifactorial process, and blood–brain barrier disruption enables the detection of circulating mediators. The clinical relevance of circulating placental growth factor (PlGF) in GBM remains unclear. This study aimed to investigate the role of PlGF in GBM and its association with disease characteristics and outcomes. Methods: We conducted a prospective observational study on 54 patients with IDH-wildtype GBM. Plasma samples collected at diagnosis and recurrence were analyzed using a multiplex panel of angiogenesis mediators. Associations with clinical, radiological, molecular, and treatment-related variables were assessed, along with survival outcomes. Statistical analysis was performed with R 4.5.0. Results: At baseline, PlGF correlated with multiple angiogenic mediators, including VEGF, IL-6, angiopoietin-1, EGF, FGF, IL-8, and TNF-α. Higher PlGF levels were associated with radiopathological features of tumor biology, including proliferation markers and the FLAIR/contrast enhancement ratio. In high-risk patients (RPA 3–4; n = 33), low baseline PlGF identified a subgroup with significantly longer overall survival (17.6 vs. 8.5 months; log-rank p = 0.031) and retained a protective association in multivariable models. In the overall cohort, this association was weaker and did not reach statistical significance. Exploratory longitudinal analyses suggested an increase in PlGF at recurrence in selected molecular and treatment-defined subgroups, while no association with bevacizumab exposure was observed. Conclusions: Circulating PlGF may reflect tumor biology in GBM and shows prognostic relevance in high-risk patients, where low baseline levels identify a subgroup with improved survival. These findings support PlGF as a candidate circulating biomarker and warrant validation in larger prospective cohorts.