• Histologically confirmed metastatic uveal melanoma.

• Progression after at least one prior line of therapy for metastatic uveal melanoma. Liver directed therapy (e.g., hepatic arterial embolization, isolated hepatic perfusion) will count as one line of therapy. Should any additional treatment(s) receive regulatory approval for metastatic uveal melanoma during the conduct of this trial, participants (if eligible for the newly approved treatment) would need to demonstrate disease progression on the additional treatment(s) before being eligible to participate in the current study. There is no limit to the number of previous treatments for metastatic disease.

• Participants must have measurable disease per RECIST 1.1.

• Adults, age 18 or over, with no upper age limit.

• ECOG (Eastern Cooperative Oncology Group) performance status of 0-1 (Karnofsky ≥ 70 percent).

• Acceptable organ and marrow function as defined below:

‣ Leucocytes ≥ 3,000/μL

⁃ Absolute neutrophil count ≥ 1,500/μL

⁃ Platelets ≥ 100,000/μL

⁃ Aspartate aminotransferase (AST)/ Alanine aminotransferase (ALT) ≤ 2.5x institutional upper limit of normal (ULN)

⁃ Bilirubin ≤ 1.5x institutional upper limit of normal (ULN)

⁃ Creatinine clearance ≥ 60mL/min/1.73m\^2 (measured by Cockcroft-Gault equation using actual body weight in kilograms, and then adjusted for body surface area)

• Male participants who are sexually active, and female participants of childbearing potential must agree to use 2 forms of FDA approved contraceptive methods during treatment with 225Ac-MTI-201 and up to 3 months following treatment.

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