Inclusion Criteria:

          -  Male or female patient aged 18 years or older.

          -  Pathologic diagnosis of DLBCL, as defined by the 2016 WHO classification, to include:
             DLBCL not otherwise specified; primary mediastinal large B-cell lymphoma; and
             high-grade B-cell lymphoma, with MYC and BCL2 and/or BCL6 rearrangements

          -  Relapsed or refractory disease following two or more multi-agent systemic treatment
             regimens

          -  Patients who have received previous CD19-directed therapy must have a biopsy that
             shows CD19 protein expression after completion of the CD19-directed therapy.

          -  Measurable disease as defined by the 2014 Lugano Classification

          -  Availability of formalin-fixed paraffin-embedded (FFPE) tumor tissue block or minimum
             10 freshly cut unstained slides if block is not available

          -  ECOG performance status 0-2

          -  Adequate organ function

          -  Negative beta-human chorionic gonadotropin (β-HCG) pregnancy test within 7 days prior
             to start of study drug (C1D1) for women of childbearing potential

          -  Women of childbearing potential must agree to use a highly effective method of
             contraception from the time of giving informed consent until at least 16 weeks after
             the last dose of loncastuximab tesirine. Men with female partners who are of
             childbearing potential must agree that they will use a highly effective method of
             contraception from the time of giving informed consent until at least 16 weeks after
             the patient receives his last dose of loncastuximab tesirine.

        Exclusion Criteria:

          -  Previous treatment with loncastuximab tesirine

          -  Known history of hypersensitivity to or positive serum human ADA to a CD19 antibody

          -  Pathologic diagnosis of Burkitt lymphoma

          -  Active second primary malignancy other than non-melanoma skin cancers, non-metastatic
             prostate cancer, in situ cervical cancer, ductal or lobular carcinoma in situ of the
             breast, or other malignancy that the Sponsor's medical monitor and Investigator agree
             and document should not be exclusionary

          -  Autologous stem cell transplant (ASCT) within 30 days prior to start of study drug
             (C1D1)

          -  Allogeneic stem cell transplant (AlloSCT) within 60 days prior to start of study drug
             (C1D1)

          -  Active graft-versus-host disease

          -  Post-transplant lymphoproliferative disorders

          -  Active autoimmune disease, including motor neuropathy considered of autoimmune origin
             and other central nervous system (CNS) autoimmune disease

          -  Known seropositive and requiring anti-viral therapy for human immunodeficiency (HIV)
             virus, hepatitis B virus (HBV), or hepatitis C virus (HCV).

          -  History of Stevens-Johnson syndrome or toxic epidermal necrolysis

          -  Lymphoma with active CNS involvement at the time of screening, including
             leptomeningeal disease

          -  Clinically significant third space fluid accumulation (i.e., ascites requiring
             drainage or pleural effusion that is either requiring drainage or associated with
             shortness of breath)

          -  Breastfeeding or pregnant

          -  Significant medical comorbidities

          -  Major surgery, radiotherapy, chemotherapy or other anti-neoplastic therapy within 14
             days prior to start of study drug (C1D1), except shorter if approved by the Sponsor

          -  Use of any other experimental medication within 14 days prior to start of study drug
             (C1D1)

          -  Planned live vaccine administration after starting study drug (C1D1)

          -  Failure to recover to Grade ≤1 (Common Terminology Criteria for Adverse Events version
             4.0 [CTCAE v4.0]) from acute non-hematologic toxicity (Grade ≤2 neuropathy or
             alopecia) due to previous therapy prior to screening

          -  Congenital long QT syndrome or a corrected QTcF interval of >480 ms at screening
             (unless secondary to pacemaker or bundle branch block)

          -  Any other significant medical illness, abnormality, or condition that would, in the
             Investigator's judgment, make the patient inappropriate for study participation or put
             the patient at risk