In the clinical use of third-line treatment of non-Hodgkin lymphoma (NHL), the combination treatment is increasingly used due to problems such as drug resistance, and while their efficacy has been proven, whether they are economical has become a new issue. A recent trial showed copanlisib plus rituximab combination therapy (CRCT) had better efficacy in the treatment of relapsed indolent NHL (iNHL) compared to rituximab monotherapy (RM). However, the long-term cost and effectiveness of this regimen is not known. We are the first to evaluate the cost effectiveness of CRCT in third-line treatment of relapsed iNHL from the perspective of US payers. We used a Markov model to evaluate cost and quality-adjusted life years (QALYs) which included a population from CHRONOS-3 with mean age of 62.5 years and total cycle length of 16.3 years. The cycle length was 1 month, adverse reaction rates were from CHRONOS-3, mean body surface area was referenced from published literature, cost values are referenced from published literature and Drugbank, utility values were referenced from the published literature, and the primary endpoint was the incremental cost-effectiveness ratio (ICER). The willingness to pay (WTP) threshold was set at $150,000 per QALYs, and one-way sensitivity analysis and probabilistic sensitivity analysis were used to verify the robustness of the model. All costs are expressed in 2021 dollars and costs and utilities have been calculated at a discount rate of 3% per year. CRCT and RM obtained 6.53 QALYs and 5.15 QALYs, respectively, and the ICER of CRCT vs. RM was $358,895.2/QALYs. Parameters having the greatest impact on the robustness of the model were the drug cost of copanlisib and the utility value of the progression-free survival (PFS) state. When the WTP threshold was $150,000, the probability of CRCT and RM being the most cost effective was 0.4% and 99.6% respectively. From a US payer perspective, CRCT is not cost-effective in treating relapsed iNHL at current prices compared to RM. But given its positive clinical efficacy, appropriate price discounts or assistance programs should be considered to make CRCT more affordable to patients with relapsed iNHL.