Abstract. Research on rental housing discrimination (RHD) against migrant minorities has overwhelmingly focused on the first generation, paying special attention to the most sizeable immigrant groups. As a result, we still know little about the housing-market experiences of immigrants’ children and the less sizeable—but often fast-growing—ethnic minorities. RHD research has also lagged behind the spectacular growth of online apps for conducting private rental transactions. We present a novel instant messaging correspondence test to study (real) private landlords’ responses to (fictitious) flat seekers of native and Chinese background in Madrid, Spain. Drawing on instant messaging allows us to introduce innovative treatments for phenotype and cultural assimilation. We find moderate levels of RHD against visibly Chinese-background applicants with a fully Chinese name and who use full Chinese characters in their WhatsApp status profile (low assimilation condition) but very low levels of discrimination against visibly identical applicants who combine a Spanish first name with a Chinese last name (typical of the second generation) and who use the word “Madrid” in Latin alphabet in their app status profile (high assimilation condition). Finally, we find adding signals of flat-seekers’ income reliability (diagnostic treatment) does not reduce discrimination propensity. Results are robust to stringent controls for ethnic composition and COVID-19 incidence rate at the district level. These findings highlight the primacy of perceived cultural assimilation over racial appearance and information deficits in shaping RHD against Chinese minorities in Spain and illustrate the analytical pay-offs of using instant messaging correspondence tests in discrimination research.