Introduction There is consistent evidence that suggests first year students are at risk of weight gain, but the reasons for this vulnerability are still unclear. Given the dramatic changes in routine, environment and social life experienced by first year undergraduate students, some level of self-regulatory skills may be required to keep healthy habits and/or build new ones due to disruptions of old habitual behaviours. The new environment may also increase demands on self-regulation to inhibit impulses towards food temptations, since students may be exposed to a high level of unhealthy food options at university. Therefore, this study aimed to examine relationships between eating self-regulatory skills, weight and dietary behaviours over 6 months in a UK cohort of undergraduate students. Methods First year undergraduate students from 13 universities situated across London were invited to complete an online survey at the beginning of the academic year (September/October 2015). At 6-month follow-up participants were invited to complete the online survey for a second time. Eating self-regulatory skills at baseline were assessed using a valid 5-item scale. Weight changes over 6 months were calculated for absolute weight in kg and categorised into two groups: ≥ 5% or Results A total of 481 students took part in the study. A mean weight change of 0.661 kg (sd = 3.83, P = 0.0006) was observed, but the range of weight change varied widely (−11.3 kg to 26.2 kg). Among students whose weight increased over 6 months (N = 127), the mean weight gain was 3.30 kg (sd 3.16) and around a quarter of participants (23.5%, N = 60) gained 5% or more of their initial body weight. The percentage of people with a high fruit and vegetable intake over 6 months did not significantly change (25.4 to 30.5%, P = 0.14). The percentage of people with a high sweet and salty snack intake increased significantly (50.1 to 59.9%, P = 0.01), while there was a significant decrease in the percentage of people with a high intake of sugary drinks (55.9 to 46%, P = 0.01). Students with higher baseline eating self-regulatory skills were more likely to maintain or achieve a higher fruit and vegetable (OR = 1.8, P = 0.007) and a lower sweet and salty snack (OR = 1.9, P = 0.001) intake over the 6 months. Higher baseline eating self-regulatory skills were also related to lower weight changes (β=−0.15, P = 0.018) and decreased likelihood of gaining 5% initial body weight (OR = 0.52, P = 0.006) at 6-month follow-up. Additionally, self-regulatory skills moderated the relationship between baseline BMI and weight changes (β=−0.25, P ≤ 0.001) and between baseline BMI and 5% weight gain (OR = 0.82, P = 0.008). Conclusions Starting university with higher eating self-regulatory skills may help students to maintain or achieve a healthy diet and protect them against substantial weight gain, especially among students with higher BMIs. Weight gain prevention initiatives that include eating self-regulatory skills training should be tested.