• Signed informed Consent Form

• Age ≥ 18 years on day of signing informed consent

• Histologically confirmed malignant pleural mesothelioma

• Surgical resection (P/D), without macroscopic residual. For stage I patient without visceral involvement a total pleurectomy is allowed

• Absence of measurable or non-measurable disease assessed with CT scan after surgery

• Patients must have received at least no 4 cycles of platinum/pemetrexed

• perioperative chemotherapy as per local practice (neoadjuvant or adjuvant or both). Less than 4 cycles of chemotherapy are allowed for clinical decisions

• \- In patients previously treated with neoadjuvant chemotherapy, randomization

• should occur within 50 days from surgical resection.

• \- In patients treated with adjuvant chemotherapy, randomization should occur

• within 30 ±7 days from last dose of adjuvant treatment.

• Performance status of 0-1 on the ECOG Performance Scale

• Availability of a representative tumor specimen for exploratory biomarker research (see Section 4.5.6 for information on tumor specimens) A formalin-fixed paraffin-embedded (FFPE) tumor specimen in a paraffin block (preferred) or at least 10 slides containing unstained, freshly cut, serial sections must be submitted along with an associated pathology report prior to study enrollment.

• Adequate hematologic and end-organ function, defined by the following laboratory test results, obtained within 14 days prior to initiation of study treatment: ANC ³ 1.5 ´ 109/L (1500/mL) without granulocyte colony-stimulating factor support Lymphocyte count ³ 0.5 ´ 109/L (500/mL) Platelet count ³ 100 ´ 109/L (100,000/mL) without transfusion Hemoglobin 9 g/dL Patients may be transfused to meet this criterion. AST, ALT, and alkaline phosphatase (ALP) £ 2.5 ´ upper limit of normal (ULN)

⁃ Bilirubin £ 1.5 ´ ULN with the following exception:

⁃ Patients with known Gilbert disease: bilirubin level £ 3 ´ ULN. Creatinine £ 1.5 ´ ULN Albumin ³ 25 g/L (2.5 g/dL) For patients not receiving therapeutic anticoagulation: INR or aPTT £ 1.5 ´ ULN

⁃ • For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraception, as defined below: Women must remain abstinent or use contraceptive methods with a failure rate of \< 1% per year during the treatment period and for 5 months after the final dose of atezolizumab. Women must refrain from donating eggs during this same period.

⁃ A woman is considered to be of childbearing potential if she is postmenarcheal, hasnot reached a postmenopausal state (³12 continuous months of amenorrhea with no identified cause other than menopause), and is not permanently infertile due to surgery (i.e., removal of ovaries, fallopian tubes, and/or uterus) or another cause as determined by the investigator (e.g., Müllerian agenesis). Examples of contraceptive methods with a failure rate of \<1% per year include bilateral tubal ligation, male sterilization, hormonal contraceptives that inhibit ovulation, hormone-releasing intrauterine devices, and copper intrauterine devices.

⁃ The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the patient. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) andn withdrawal are not acceptable methods of contraception. If required per local guidelines or regulations, locally recognized acceptable methods of contraception and information about the reliability of abstinence will be described in the local Informed Consent Form.

⁃ Women with a positive pregnancy test at enrollment or prior to administration of study medication will be excluded.