Inclusion Criteria:
* Histopathologically or molecularly (per c-IMPACT NOW criteria) proven diagnosis of glioblastoma which is recurrent following radiation therapy (prior dose must have been between 40 and 75 Gy).
* Tumor O-6-methylguanine-deoxyribonucleic acid (DNA) methyltransferase (MGMT) methylation status must be available from any prior GBM tumor specimen.
* Patients must have measurable contrast-enhancing disease (defined by at least 1cm x 1cm) by magnetic resonance imaging (MRI) imaging within 28 days of starting study treatment.
* Patients may have had treatment for an unlimited number of prior relapses.
* Patients must have recovered from severe toxicity of prior therapy.
* Patients must be able to swallow oral medications.
* Karnofsky performance status \>= 60.
* Life expectancy \>3 months.
* Adequate hematologic parameters.
* Adequate hepatic function within 7 days prior to start of study treatment.
* Adequate renal function within 7 days prior to start of study treatment.
* Reproductive Status
* Women - negative serum or urine pregnancy test
* Men and Women - must agree to an adequate method to avoid pregnancy
* Participant must agree to not donate blood during the study or for 90 days after the last dose of niraparib.
* Participant must, in the opinion of the Investigator, be able to comply with study procedures, including use of the Optune device.
* Cohort B (surgical) patients only: patients must be undergoing surgery that is clinically indicated as determined by their care providers.
* Cohort B (surgical) patients only: patients must have a tumor tissue form indicating availability of archived tissue from a previous surgery for glioblastoma.
* Patients must be able to understand the study procedures and agree to participate in the study by providing written informed consent (or have legally authorized representative sign on patient's behalf if patient physically unable to sign consent due to neurologic deficit).
Exclusion Criteria:
* Age \< 22 years.
* Prior treatment with tumor-treating fields therapy (Optune) within the past 6 months.
* Prior treatment with a PARP inhibitor.
* Known history or current diagnosis of myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML).
* Patients with infratentorial tumor.
* Participant has had any known Grade 3 or 4 anemia, neutropenia or thrombocytopenia due to prior chemotherapy that persisted \> 4 weeks and was related to the most recent treatment.
* Participant must not have a serious, uncontrolled medical disorder, nonmalignant systemic disease, or active, uncontrolled infection. Examples include, but are not limited to, uncontrolled ventricular arrhythmia, recent (within 90 days) myocardial infarction, uncontrolled major seizure disorder, unstable spinal cord compression, superior vena cava syndrome, or any psychiatric disorder that prohibits obtaining informed consent.
* Implanted pacemaker, defibrillator or deep brain stimulator, other implanted electronic devices in the brain. Non-programmable shunts are allowed. Patients with a programmable shunt are excluded.
* Skull defects.
* Known hypersensitivity to conductive hydrogels or known hypersensitivity to niraparib components or excipients.
* Patients with gastrointestinal disorders or abnormalities that would interfere with absorption of study treatment.
* Prisoners or subjects who are involuntarily incarcerated.
* Subjects who are compulsorily detained for treatment of either a psychiatric or physical (eg, infectious disease) illness.
* Participant must not be simultaneously enrolled in any interventional clinical trial.
