Inclusion Criteria:

   - Histopathological confirmation of locally advanced unresectable or metastatic
   urothelial carcinoma (LA/mUC), including UC originating from the renal pelvis,
   ureters, bladder, or urethra.

   - Measurable disease by investigator assessment per RECIST v1.1.

   - Participant must not have received prior systemic therapy for LA/mUC. Exception will
   be made for neoadjuvant or adjuvant therapy, if disease recurrence/progression
   occurred more than 12 months after the last dose of therapy.

   - Eligible to receive cisplatin- or carboplatin-containing chemotherapy.

   - Able to provide archived formalin-fixed paraffin-embedded tumor tissue blocks from a
   muscle-invasive or metastatic UC lesion or biopsy of metastatic UC prior to treatment
   initiation. If archival tissue is not available a newly obtained baseline biopsy of an
   accessible tumor lesion is required within 28 days of cycle 1 day 1.

   - HER2 expression of 1+ or greater on immunohistochemistry (IHC).

   - Eastern Cooperative Oncology Group (ECOG) performance score of 0, 1, or 2 within 7
   days prior to randomization.

Exclusion Criteria:

   - Known hypersensitivity to disitamab vedotin, cisplatin, carboplatin, gemcitabine, or
   pembrolizumab or any of their components.

   - History of severe/life threatening immune-related adverse event (irAE) with PD-(L)1
   inhibitors are excluded.

   - Central nervous system (CNS) and/or leptomeningeal metastasis. Participants with
   treated CNS metastases are permitted if all of the following are met.

      - CNS metastases have been clinically stable for at least 4 weeks and baseline
      scans show no evidence of new or worsening CNS metastasis.

      - Participant is on a stable dose of ≤ 10 mg/day of prednisone or equivalent for at
      least 2 weeks.

   - History of or active autoimmune disease that has required systemic treatment in the
   past 2 years.

   - Prior treatment with an agent directed to another stimulatory or co-inhibitory T cell
   receptor (including but not limited to CD137 agonists, CAR-T cell therapy, CTLA-4
   inhibitors, or OX-40 agonists).

   - Prior solid organ or bone marrow transplantation.

   - Pleural effusion or ascites with symptoms or requiring symptomatic treatment.

   - Estimated life expectancy <12 week

   - Prior treatment with an MMAE agent or anti-HER2 therapy