• 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