• Participants must have endometrial cancer that is ARID1A-mutated \[loss of function (LOF) mutations\] determined by any CLIA-certified next-generation sequencing assay. ARID1A LOF mutation status must be confirmed by the principal investigator prior to participant enrollment.

• Participants must have measurable disease per RECIST 1.1, defined as at least one lesion that can be accurately measured in at least one dimension (longest diameter to be recorded for non-nodal lesions and short axis for nodal lesions). Each lesion must be \>= 10 mm when measured by CT, MRI or caliper measurement by clinical exam; or \>= 20 mm when measured by chest x-ray. Lymph nodes must be \> 15 mm in short axis when measured by CT or MRI.

• Prior Therapy: There is no upper limit of prior therapies, but patients must have had one prior chemotherapeutic regimen for management of endometrial carcinoma. Initial treatment may include chemotherapy, chemotherapy and radiation therapy, and/or consolidation/maintenance therapy. Furthermore, patients who have only received chemotherapy with immunotherapy in the adjuvant setting will be eligible for the study.

⁃ -Prior hormonal therapy is allowed (no washout period is required after hormonal therapy).

• Participants must have received prior immunotherapy targeting the PD-1/PD-L1 pathway either in the adjuvant, first-line or recurrent setting. Up to 50% of participants (i.e. a maximum of 12 participants) who do not meet this requirement may enroll in the study.

• Age ≥18 years. Because no dosing or adverse event data are currently available on the use of the combination of avelumab and M1774 in participants \<18 years of age, children are excluded from this study.

• ECOG performance status 0, 1, or 2 (reference Appendix A for ECOG performance status criteria).

• Availability of a formalin fixed paraffin embedded (FFPE) block of cancer tissue OR 15 unstained 5-micron slides from the original surgery or biopsy or from a biopsy of recurrent disease.

• Participants must meet the following organ and marrow function as defined below:

‣ Absolute neutrophil count ≥ 1,500/mcL

⁃ Hemoglobin ≥ 9.0 g/dL (with no erythropoietin or red blood cell transfusion within the last 14 days)

⁃ Platelets ≥ 100,000/mcL

⁃ Total bilirubin ≤ 1.5 institutional upper limit of normal (ULN) in the case of documented Gilbert's syndrome, total bilirubin ≤3 x ULN is allowed

⁃ AST(SGOT)/ALT(SGPT) ≤ 2.5 × institutional ULN

⁃ Creatinine clearance ≥ 60 mL/min as estimated by the Cockcroft-Gault formula or institutional standard method OR

⁃ Glomerular filtration rate (GFR) ≥ 60 mL/min by measured 24-hour urine collection

• The effects of avelumab and M1774 on the developing human fetus are unknown. For this reason and because these agents are known to be teratogenic, women of child-bearing potential must have a negative serum pregnancy test at screening. Women of child- bearing potential must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) prior to study entry, for the duration of study participation, and for at least 30 days after last avelumab treatment administration and at least 6 months after last M1774 treatment administration. Should a woman become pregnant or suspect she is pregnant while she is participating in this study, she should inform her treating physician immediately.

⁃ -Women of child-bearing potential are defined as those who are not surgically sterile (i.e., bilateral tubal ligation or salpingectomy, bilateral oophorectomy, or total hysterectomy) or post-menopausal (defined as ≥ 12 months with no menses without an alternative medical cause)

• Ability to understand and the willingness to sign a written informed consent document.