Study to Evaluate Adverse Events and Change in Disease Activity in Adult Participants With B-Cell Malignancies Receiving Oral ABBV-525 Tablets

Inclusion Criteria: * Dose Escalation (Part 1) Only: Participants with a documented diagnosis of one of the following third line or later of treatment (3L)+ mature B-cell malignancies, from the World Health Organization (WHO)-defined histologies as defined in the protocol. * Dose Optimization (Part 2) Only: Participants with documented diagnosis of chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma (SLL) with histology based on WHO criteria, with measurable disease requiring treatment as defined by the International Workshop on Chronic Lymphocytic Leukemia (iwCLL). * Dose Expansion (Part 3) Only: Participants with documented diagnosis of one of the 3L+ mature B-cell malignancies based on WHO criteria listed in the protocol, with measurable disease requiring treatment. * Participant has an Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) of 0, 1, or 2. * Participant has a life expectancy \>= 12 weeks. * Adequate hematological and hepatic function as defined in the protocol. * Must have archival or freshly collected tumor tissue for correlative studies before study enrollment. * Participants with prior central nervous system (CNS) disease that has been effectively treated may be eligible. * Participants with resolved coronavirus disease 2019 (COVID-19) infection are eligible. Exclusion Criteria: * Known active CNS disease, or primary CNS lymphoma. * Known bleeding disorders. * Known history of stroke or intracranial hemorrhage within 12 months prior to first dose of study treatment. * Uncontrolled active systemic infection, or active cytomegalovirus infection. * Active and/or chronic hepatitis B or C infection and/or the criteria listed in the protocol. * Known history of human immunodeficiency virus (HIV). * Known active COVID-19 infection. Participant must not have signs/symptoms associated with COVID-19 infection or known exposure to a confirmed case of COVID-19 infection during screening. If participant has signs/symptoms suggestive of COVID-19 infection, the participant must have a negative molecular (e.g., polymerase chain reaction) test or 3 negative antigen test results at least 24 hours apart.