Marginal zone lymphoma (MZL) is challenging to treat, with many patients relapsing following initial treatment. We report the long-term efficacy and safety of copanlisib, a pan-class I phosphoinositide 3-kinase (PI3K) inhibitor, in the subset of 23 patients with relapsed/refractory MZL treated in the phase 2 CHRONOS-1 study (#NCT01660451, Part B; Patients had a median of 3 prior lines of therapy, including rituximab and alkylating agents, and received IV copanlisib 60 mg on days 1, 8, and 15 of 28-day cycles for a median of 23 weeks. The objective response rate was 78.3% (18/23; 3 complete responses and 15 partial responses). The median duration of response was 17.4 months (median follow-up, 9.4 months), and median time to response was 2.1 months. Median progression-free survival was 24.1 months (median follow-up, 10.3 months), and median overall survival was not reached (median follow-up, 28.4 months). The most common all-grade treatment-emergent adverse events (TEAEs) included fatigue (52.2%, 12/23), diarrhea, and transient, infusion-related hyperglycemia (each 47.8%, 11/23). Nineteen patients (82.6%) had grade 3/4 TEAEs, most commonly transient, infusion-related hyperglycemia and hypertension (each 39.1%, 9/23). TEAEs led to dose reduction or dose interruptions /delays in 9 patients (39.1%) and 18 patients (78.3%), respectively. Patients with activated PI3K/B-cell antigen receptor signaling had improved response rates. Overall, copanlisib demonstrated strong efficacy, with a short time to objective response, improved objective response rate with longer treatment duration, durable responses, and manageable safety, in line with previous reports. These data provide rationale for long-term treatment with copanlisib in patients with relapsed/refractory MZL.