• The participant (or legally acceptable representative) has provided documented informed consent for the study.

• Be ≥ 18 years of age on day of providing informed consent.

• Participant with new diagnosis of GBM according to World Health Organization (WHO) 2021 Classification.

• Recovered from maximal debulking surgery (gross total resection, partial resection and biopsy-only patients are all acceptable), Gliadel wafers placement at the time of surgical resection is not allowed.

• Have completed standard adjuvant chemoradiotherapy of radiotherapy (RT) according to local practice (56-64 Gy), and concomitant TMZ chemotherapy.

• Amenable to treatment with Optune concomitant with maintenance TMZ (150-200 mg/m\^2 daily x 5, Q28 days).

• Have an Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 to 1 assessed within 7 days before randomization.

• Stable or decreasing dose of corticosteroids (dexamethasone ≤ 2mg or equivalent) for the last 7 days prior to randomization, if applicable.