Abstract Background Atypical and anaplastic meningiomas account for 20% of all meningioma cases. Solitary fibrous tumor (SFT) is a type of soft tissue sarcoma with similar attributes to meningioma. For patients with refractory or recurrent disease after previous surgery or radiotherapy, there is no effective treatment. Pembrolizumab, an anti-programmed cell death 1 (PD1) antibody, is an effective treatment for various solid tumors. PD1 ligand is highly expressed in aggressive meningiomas. We aimed to assess the effectiveness of pembrolizumab in treating meningioma and SFT recurrence after surgery and radiation therapy. Methods This prospective single-arm phase II trial comprised 15 patients with recurrent meningioma and 3 with anaplastic SFT, treated at a single institution during 2018-2022. The study was terminated due to lack of efficacy and slow accrual. The primary endpoint was 6-month progression-free survival (PFS-6). Results Median PFS was 2.6 months, and median overall survival (OS) was 40 months. Six and 12 months PFS were both 11.1%. The 6- and 12-month OS were 94.4% and 61.1%, respectively. According to the Response Assessment in Neuro-Oncology (RANO) criteria, the overall response rate was 11%, with 2 patients achieving stable disease and 2 with partial response. Three patients (16.7%) developed grade 3 toxicity. Conclusions Our results showed that pembrolizumab failed to improve PFS-6 in patients with aggressive meningioma or anaplastic SFT. However, two patients, one with atypical meningioma and one with anaplastic SFT, achieved partial response. More clinical studies are needed to identify which subset of patients may benefit from this treatment.