Aim: Despite growing interest in the relationship between social media use, dietary behaviors, and obesity-related indicators, the cognitive and perceptual factors underlying these relationships are still not sufficiently understood. This study aimed to examine the association between social media use and body mass index (BMI) and waist circumference (WC), within the framework of the sequential mediating roles of health-oriented food choice motives (HO-FCM) and self-rated diet quality (SRDQ). Subjects and Method: In this cross-sectional study, sociodemographic data and data on SRDQ, the Social Media Usage Scale, and the Food Choice Questionnaire (FCQ) were collected from 2727 adult women living in Istanbul, and anthropometric measurements were obtained. HO-FCM were derived from the health, body weight control, and natural content subscales of the FCQ. Serial mediation analyses (PROCESS Model 6) were conducted with adjustment for sociodemographic and health-related variables. Results: Among participants’ food choice motives, sensory appeal ranked highest, whereas body weight control and ethical concern ranked lowest. Social media use showed no significant direct association with BMI or WC. However, it showed a significant positive association with HO-FCM (b = 0.0078, p < 0.001) and a significant negative association with SRDQ (b = −0.0100, p < 0.001). Mediation analyses indicated a positive indirect effect through SRDQ (BMI: b = 0.0049, 95% CI [0.0024, 0.0081]; WC: b = 0.0155, 95% CI [0.0078, 0.0248]) and a significant negative serial indirect effect through HO-FCM and SRDQ (BMI: b = −0.0008, 95% CI [−0.0014, −0.0003]; WC: b = −0.0025, 95% CI [−0.0043, −0.0011]). The total indirect effects were positive and significant for both BMI (b = 0.0052, 95% CI [0.0019, 0.0090]) and WC (b = 0.0168, 95% CI [0.0071, 0.0279]). Conclusion: The findings suggest that social media use may be associated with anthropometric indicators through motivational and perceptual factors among adult women. Accordingly, longitudinal studies that address different social media platforms and content types separately and incorporate objective dietary assessments are thought to be needed.