This pre-registered secondary analysis aimed to examine distinct longitudinal loneliness trajectories in youth and whether these trajectories were associated with psychological distress at final follow-up in the UK Household Longitudinal Study. Participants (N = 827, 55.1% female, Time 1: M ± SD = 16.50 ± 0.50 years) provided data during Waves 9, 10 and 11. K-means longitudinal clustering analysis was used to identify clusters of participants with distinct loneliness trajectories across measurement waves. We identified four clusters demonstrating distinct trajectories of loneliness: stable low (40.7%), stable high (20.6%), moderate decreasing (19.6%) and low increasing (19.1%). Compared to 'stable low loneliness', 'stable high' and 'low increasing' loneliness clusters were significantly associated with psychological distress at Wave 11 following adjustment for sex, ethnicity, parent's highest educational achievement and Wave 9 psychological distress. The current study offers an important contribution to the literature on patterns of youth loneliness and mental health consequences.