Dominant theoretical discussions on insider/outsider, co-ethnic/co-national migrant researcher positionalities have focused on the ideas of group identities such as nationality and ethnicity and how they shape and inform insider/outsider researchers’ positionalities. While some migration researchers argue that shared nationality and ethnicity make co-national or co-ethnic researchers insiders, others contend that the insiderness/outsiderness of co-national or co-ethnic researchers tends to be shaped and informed by multiple, fluid and changing situational factors. This paper draws on ‘fluid identity theory’ and secondary literature to argue that in migration research, insider/outsider positionalities tend to be fluid formations that change, shift and become unstable during research encounters with study participants. I develop an analytical concept that I term ‘liquid insider/outsider positionalities’ to contribute to the literature on insider/outsider researcher positionalities in migration research. By way of introducing this analytical concept, I critique presuppositions, conceptualizations and categorizations of migrant/migration researchers as either insiders or outsiders based on predetermined and rigid social identity markers such as ethnicity or nationality. Migration scholars and researchers may employ the concept of ‘liquid insider/outsider positionalities’ as a tool to frame the dynamic, changing and situational character of researcher positionalities in migration research during field research encounters.