Abstract Objective This study investigated cross‐sectional and longitudinal associations of fast‐food outlet exposure with BMI and BMI change, as well as moderation by age and genetic predisposition. Methods This study used Lifelines' baseline ( n = 141,973) and 4‐year follow‐up ( n = 103,050) data. Participant residential addresses were linked to a register with fast‐food outlet locations (Nationwide Information System of Workplaces [Dutch: Landelijk Informatiesysteem van Arbeidsplaatsen, LISA]) using geocoding, and the number of fast‐food outlets within 1 km was computed. BMI was measured objectively. A weighted BMI genetic risk score was computed, representing overall genetic predisposition toward elevated BMI, based on 941 single‐nucleotide polymorphisms genome‐wide significantly associated with BMI for a subsample with genetic data (BMI: n = 44,996; BMI change: n = 36,684). Multivariable multilevel linear regression analyses and exposure‐moderator interactions were tested. Results Participants with ≥1 fast‐food outlet within 1 km had a higher BMI (B [95% CI]: 0.17 [0.09 to 0.25]), and those with ≥2 fast‐food outlets within 1 km increased more in BMI (B [95% CI]: 0.06 [0.02 to 0.09]) than participants with no fast‐food outlets within 1 km. Effect sizes on baseline BMI were largest among young adults (age 18–29 years; B [95% CI]: 0.35 [0.10 to 0.59]) and especially young adults with a medium (B [95% CI]: 0.57 [−0.02 to 1.16]) or high genetic risk score (B [95% CI]: 0.46 [−0.24 to 1.16]). Conclusions Fast‐food outlet exposure was identified as a potentially important determinant of BMI and BMI change. Young adults, especially those with a medium or high genetic predisposition, had a higher BMI when exposed to fast‐food outlets.