Chinese rural-to-urban migrant workers (MWs) who are employed in service industry are a rapidly growing population in urban China. Like other MWs, service industry MWs (SIMWs) are generally excluded from the mainstream of city societies, but unlike other MWs, they are more marginalized in cities. Social isolation increases the feelings of loneliness; however, there are little empirical data on the epidemiology of loneliness of SIMWs. The present study aimed to investigate the prevalence and associated factors of loneliness among SIMWs in Shenzhen, China. By using respondent-driven sampling, 1979 SIMWs were recruited and administered with standardized questionnaires to collect data on sociodemographics, physical health, and migration-related characteristics. Loneliness and social support were measured with a single-item self-report question "Do you feel lonely often?" and Multidimensional Scale of Perceived Social Support (MSPSS), respectively. 18.3% of SIMWs reported feeling lonely often. Being aged 60 years or older (odds ratio [OR] = 2.30), marital status of "others" (OR = 2.77), being physically ill in the last 2 weeks (OR = 1.46), migrating alone (OR = 1.97), working >8 hours/day (OR = 1.06), MSPSS inside family subscale score ≤18 (OR = 1.80), and MSPSS outside family subscale score ≤38 (OR = 1.50) were significantly associated with increased risk of loneliness in SIMWs. Loneliness is prevalent in Chinese SIMWs and should be seen as a major public health issue. The high prevalence and many negative health consequences of loneliness highlight the importance of routine screening, evaluation, and treatment of loneliness in this vulnerable population.