Inclusion Criteria:

1. * Confirmed diagnosis of CLL/SLL requiring therapy per iwCLL 2018 criteria
2. * Eastern Cooperative Oncology Group (ECOG) Performance Status 0-2
3. * Adequate organ function

1. * Platelets greater than or equal to (≥)50 x 10⁹/liter (L) or ≥30 x 10⁹/L in participants with documented bone marrow involvement considered to impair hematopoiesis,
2. * Hemoglobin ≥8 grams/deciliter (g/dL) or ≥6 g/dL in participants with documented bone marrow involvement considered to impair hematopoiesis
3. * Absolute neutrophil count ≥0.75 x 10⁹/L or ≥0.50 × 10⁹/L in participants with documented bone marrow involvement considered to impair hematopoiesis
4. * Kidney function: Estimated creatinine clearance ≥30 milliliters per minute (mL/min)

Exclusion Criteria:

1. * Known or suspected Richter's transformation to diffuse large B-cell lymphoma (DLBCL), prolymphocytic leukemia, or Hodgkin's lymphoma at any time preceding enrollment
2. * Known or suspected central nervous system (CNS) involvement
3. * A significant history of renal, neurologic, psychiatric, endocrine, metabolic or immunologic disease
4. * Active uncontrolled auto-immune cytopenia (e.g., autoimmune hemolytic anemia \[AIHA\], idiopathic thrombocytopenic purpura \[ITP\])
5. * Significant cardiovascular disease including ejection fraction \< 40% and any grade ongoing atrial fibrillation or atrial flutter
6. * Hepatitis B or hepatitis C testing indicating active/ongoing infection, based on Screening laboratory tests
7. * Active cytomegalovirus (CMV) infection
8. * Active uncontrolled systemic bacterial, viral, or fungal infection
9. * Known human immunodeficiency virus (HIV) infection, regardless of cluster of differentiation 4 (CD4) count
10. * Clinically significant active malabsorption syndrome or other condition likely to affect GI absorption of the oral-administered study treatments
11. * Ongoing inflammatory bowel disease
12. * Prior exposure to BTK inhibitor (covalent or noncovalent)
13. * Concurrent use of investigational agent or anticancer therapy except hormonal therapy
14. * Participants requiring therapeutic anticoagulation with warfarin or another Vitamin K antagonist
15. * Use of ≥ 20 mg prednisone daily or equivalent dose of steroid at the time of first dose of study drug
16. * Vaccination with a live vaccine within 28 days prior to randomization
17. * Participants receiving chronic therapy with a strong cytochrome P450 (CYP)3A inhibitor (except posaconazole and voriconazole) which cannot be stopped within 3-5 half lives of the CYP3A inhibitor therapy prior to start of study drug treatment
18. * Participants with known hypersensitivity, including anaphylaxis, to any component or excipient of pirtobrutinib or ibrutinib