Background: MZL is a rare indolent B-cell malignancy considered incurable at recurrent stage. Bruton tyrosine kinase (BTK) inhibitors have produced durable responses in patients (pts) with R/R MZL. Acalabrutinib (acala) is a potent next-generation BTK inhibitor with high selectivity for BTK. We report data for acala monotherapy from the R/R MZL cohort (phase 2) of a phase 1b/2 clinical trial (NCT02180711). Methods: Pts with histologically confirmed MZL, ECOG performance status ≤2, and ≥1 prior therapy (including ≥1 CD20-directed regimen) received oral acala 100 mg twice daily until disease progression or unacceptable toxicity ± R. The primary objective was overall response rate (ORR; Lugano criteria as assessed by the investigator). Secondary objectives were duration of response (DOR), progression-free survival (PFS), overall survival (OS), and safety. Data were analyzed descriptively (no formal hypothesis testing). Results: Forty-two pts received acala (median age 69 y [range 42–84]; median 2 prior systemic regimens [range 1–4]). MZL subtypes were extranodal (43%), nodal (31%), and splenic (26%). At data cutoff (Oct 15, 2021), median follow-up duration was 10.7 mo (range 0.4–42.8). Sixteen (38%) pts discontinued acala, most commonly due to disease progression (26%). Among pts evaluable for response (n = 37; 3 pts had not reached the first assessment timepoint and 2 pts exited the study without response assessment), ORR was 54% (95% CI 37%–71%) with 6 complete (16%) and 14 partial (38%) responses; 17 (46%) pts had stable disease. ORRs in extranodal, nodal, and splenic subtypes were 65%, 44%, and 45%, respectively. Median time to initial response was 3.0 mo; median DOR was 19.3 mo (95% CI 8.4–not estimable). Median PFS was 27.4 mo with a 12-mo PFS rate of 66%. Four pts died (disease progression, n = 2; transformation to diffuse large B-cell lymphoma after stopping treatment, n = 1; adverse event [AE], n = 1 [septic shock unrelated to treatment]); median OS was not reached. Treatment was well tolerated with most AEs being grade 1 or 2. Sixteen pts (38%) had grade ≥3 AEs, most commonly (in ≥2 pts) anemia, dyspnea, neutrophil count decrease (n = 3 each), fatigue, thrombocytopenia, and neutropenia (n = 2 each). AEs led to treatment discontinuation in 2 pts (grade 3 hypotension and grade 1 myalgia). Among AEs of clinical interest, hypertension was reported in 2 pts (both grade 2); no cases of atrial fibrillation/flutter or major hemorrhage were reported. Conclusions: These early results indicate that acala is efficacious and well tolerated in pts with R/R MZL. The AE profile is consistent with the known safety profile of acala. Clinical trial information: NCT02180711.