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).

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 …