This paper investigates the morphological ergative marking in Monsang, an endangered Northwestern language of the South-Central branch of the Tibeto-Burman language family, spoken in Chandel district of Manipur, India. The paper shows that the A arguments of transitive verbs are obligatorily marked by the ergative marker -in in some contexts and in others only optionally. Factors determining ergative marking include animacy hierarchy, discourse-pragmatic focus and high agency verbs. We also observed that S arguments of unergative verbs can take optional ergative marking when the subordinate clause has the adverbial marker lè 'while/when'. The ergative marker is used for various reasons, such as, to give a reading of agency to inanimate subjects, to place focus on the agent in a discourse, to emphasize the agency of the agent. The results of the study contribute to the description of optional ergative marking in Tibeto-Burman languages in general and South-Central Tibeto-Burman languages in particular. © 2020 The Linguistic Association of Finland. All rights reserved.