In Ethiopia, the facts of farmers’ indigenous knowledge-based cropping systems have rarely been investigated through research. A field experiment was conducted during 2021/2022 main cropping season at Fogera Plain with the objective of examining the effect of additive series relay intercropping of grass pea with lowland rice on the grain yield of the component crops and the production efficiency of the cropping system. The experiment consisted of a factorial combination of four seed proportions of grass pea (SPGP) (25%, 50%, 75%, and 100% of the recommended seed rate of sole grass pea) relay intercropped with full seed rates of rice in four rice: grass pea spatial arrangements (SA) (1:1, 2:1, 3:1, and their mixed relay intercropping system). The treatments were arranged in a randomised complete block design with three replications. Data on grain yields of the component crops were collected and analysed using SAS-JMP-16 software. Results revealed that SPGP and SA had no significant effect on rice. The highest grain yield of grass pea was obtained when 25% SPGP was relay intercropped with rice in 1:3 SA (5.10 t ha-1). Maximum production efficiency in terms of total land output yield (9.89 t ha-1) and land use efficiency (ATER = 1.33), net benefit(33, 5176.79 Birr ha-1), marginal rate of return (21,428%), and positive monetary advantage index with lower competitive ratio was obtained when 50% SPGP was relay intercropped with rice in 1:3 SA. Thus, this mixture seems to contribute to the development of sustainable crop production with a limited use of external inputs. Rice intercropping with other staple legume crops under residual soil moisture needs to be tested across locations and years to intensify the production efficiency and profitability of the cropping system.