Inclusion Criteria:

          -  Pathologically confirmed locally advanced or metastatic urothelial carcinoma by
             histology

          -  Measurable disease by Response Evaluation Criteria in Solid Tumors (RECIST) version
             (v)1.1

          -  Not eligible to receive cisplatin-based chemotherapy due to renal dysfunction (defined
             as creatinine clearance [CrCl] =< 60 mL/min), > grade 2 peripheral neuropathy, or
             ototoxicity (defined as >= grade 2 hearing loss); OR unwillingness of patient to
             receive cisplatin; OR progressed on one platinum-based chemotherapy regimen for
             advanced disease

          -  May have had neoadjuvant or adjuvant platinum-based chemotherapy or intravesical
             therapy in the past

          -  >= 18 years of age

          -  Eastern Cooperative Oncology Group (ECOG) performance status score of 0, 1 or 2

          -  Life expectancy >= 3 months

          -  Absolute neutrophil count (ANC) >= 1.5 × 10^9/L

          -  Platelet count >= 100 × 10^9/L

          -  Hemoglobin >= 9 g/dL (may have been transfused)

          -  Total bilirubin level =< 1.5 x the upper limit of normal (ULN) range (except patients
             with Gilbert's syndrome who must have a total bilirubin of < 3 x ULN)

          -  Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels =< 2.5 x
             ULN, or AST and ALT levels =< 5 x ULN (for subjects with documented metastatic disease
             to the liver)

          -  Creatinine clearance >= 30 mL/min by Cockcroft-Gault formula

          -  Subjects with active hepatitis B virus (Hep B) are allowed if antiviral therapy for
             hepatitis B has been given for > 8 weeks and viral load is < 100 IU/mL prior to first
             dose of trial treatment. Subjects with untreated hepatitis C virus (HCV) are allowed.
             Subjects with human immunodeficiency virus (HIV) who have CD4+ T-cell counts >= 350
             cells/uL and no history of acquired immunodeficiency syndrome (AIDS)-defining
             opportunistic infections in the last year are allowed

          -  Willingness to undergo mandatory pre-treatment biopsy (unless there is adequate
             archival tumor specimen available for PD-L1 IHC evaluation)

          -  Female subjects who are of non-reproductive potential (i.e., post-menopausal by
             history - no menses for >= 1 year; OR history of hysterectomy; OR history of bilateral
             tubal ligation; OR history of bilateral oophorectomy). Or, female subjects of
             childbearing potential must have a negative serum or urine pregnancy test within 72
             hours prior to the first study drug administration

          -  Male and female subjects who are of reproductive potential must agree to use highly
             effective method of birth control (e.g., implants, injectables, birth control pills
             with two hormones, intrauterine devices [IUDs], complete abstinence or sterilized
             partner, and female sterilization) and a barrier method (e.g., condoms, vaginal ring,
             sponge, etc.) during the period of therapy and for 4 months after the last dose of
             study drug

          -  Ability to understand and willingness to sign an informed consent form

          -  Ability to adhere to the study visit schedule and other protocol requirements

          -  Must be able to swallow study drug

        Exclusion Criteria:

          -  Receiving radiation =< 14 days prior to enrollment to the site of selected target
             lesions

          -  Systemic therapy for cancer =< 21 days prior to enrollment

          -  Autoimmune disorder requiring active therapy as defined by corticosteroids at a dose
             >= 10 mg oral prednisone or the equivalent or requiring chronic immunosuppressive
             therapy

          -  Use of corticosteroids =< 14 days prior to enrollment at a dose of >= 10 mg oral
             prednisone or the equivalent per day

          -  Received immune checkpoint inhibitor therapy (anti-PD-1, anti-PD-L1, or anti-CTLA4
             directed therapy) on a prior clinical trial

          -  Has received selinexor or another XPO1 inhibitor previously

          -  Any active gastrointestinal dysfunction that could interfere with absorption of study
             treatment in the opinion of the investigator

          -  Pregnant or lactating women

          -  Any condition that would prohibit the understanding or rendering of informed consent

          -  Any condition including additional malignancies, laboratory abnormalities, or
             psychiatric illness that in the opinion of the investigator would interfere with the
             patient's safety or compliance while on trial

          -  Severe infection that in the opinion of the investigator would interfere with patient
             safety or compliance on trial within 4 weeks prior to enrollment