A Study of Nemtabrutinib Plus Venetoclax vs Venetoclax + Rituximab (VR) in Second-line (2L) + Relapsed/Refractory (R/R) Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma (CLL/SLL) (MK-1026-010/BELLWAVE-010).

The purpose of this study is to assess the safety and tolerability and to confirm the dose of nemtabrutinib in combination with venetoclax in participants with R/R CLL/SLL. The primary study hypotheses are that the combination of nemtabrutinib plus venetoclax is superior to VR with respect to progression-free survival (PFS) per 2018 International Workshop on Chronic Lymphocytic Leukemia (iwCLL) criteria as assessed by blinded independent central review (BICR).

Inclusion Criteria: * Confirmed diagnosis of chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) and active disease clearly documented to initiate therapy * Deletion (Del) (17p) status, tumor protein 53 (TP53) mutation status, and immunoglobulin heavy chain gene (IGHV) mutation status results required before randomization for Part 2 participants only * Relapsed or refractory to at least 1 prior available therapy * Have at least 1 marker of disease burden * Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2 within 7 days before randomization * Has a life expectancy of at least 3 months * Has the ability to swallow and retain oral medication * Participants who are hepatitis B surface antigen (HBsAg) positive are eligible if they have received hepatitis B virus (HBV) antiviral therapy for at least 4 weeks and have undetectable HBV deoxyribonucleic acid (DNA) viral load before randomization * Participants with history of hepatitis C virus (HCV) infection are eligible if HCV ribonucleic acid (RNA) viral load is undetectable at screening * Participants with human immunodeficiency virus (HIV) who meet ALL eligibility criteria * Participants with adequate organ function with specimens collected within 7 days before the start of study intervention * If capable of producing sperm, participant agrees to eliminate Nemtabrutinib: 12 days, Venetoclax: 1 month (30 days), Rituximab (rituximab biosimilar): not applicable; abstains from penile-vaginal intercourse as their preferred and usual lifestyle; OR uses prescribed contraception * Participant assigned female sex at birth are eligible to participate if not pregnant or breastfeeding and are not a person of childbearing potential (POCBP) OR is a POCBP and uses a contraceptive method that is highly effective, has a negative highly sensitive pregnancy test, and abstains from breastfeeding Exclusion Criteria: * Has an active hepatitis B virus/ hepatitis C virus (HBV/HCV) infection * Has gastrointestinal (GI) dysfunction that may affect drug absorption * Has a known additional malignancy that is progressing or has required active treatment within the past 2 years * Has diagnosis of Richter Transformation or active central nervous system (CNS) involvement by CLL/SLL * Has an active infection requiring systemic therapy, such as intravenous (IV) antibiotics, during screening * HIV-infected participants with a history of Kaposi's sarcoma and/or Multicentric Castleman's Disease and/or acquired immune deficiency syndrome (AIDS)-defining opportunistic infection in the past 12 months before screening * Clinically significant cardiovascular disease * Has a known allergy/sensitivity to nemtabrutinib or contraindication to venetoclax/rituximab (or rituximab biosimilar), or any of the excipients * Has history of severe bleeding disorders (eg, hemophilia) * Has received prior systemic anticancer therapy within 5 half-lives or 4 weeks (if prior therapy was a monoclonal antibody) before randomization * Has received prior B-cell lymphoma 2 inhibitor(s) (BCL2i) within ≤ 12 months before randomization or has received prior radiotherapy within 2 weeks of start of study intervention, or radiation related toxicities, requiring corticosteroids * Is currently being treated with p-glycoprotein (P-gp) substrates with a narrow therapeutic index, cytochrome P450 3A (CYP3A) strong or moderate inducers or CYP3A strong inhibitors. * Has received a live or live attenuated vaccine within 30 days before the first dose of study intervention * Has received an investigational agent or has used an investigational device within 4 weeks before study intervention administration * Has a known psychiatric or substance use disorder that would interfere with the participant's ability to cooperate with the requirements of the study * Participants who have not adequately recovered from major surgery or have ongoing surgical complications