Background: Children are presenting in greater numbers to gender clinics around the world. Prospective longitudinal research is important to better understand outcomes and trajectories for these children. This systematic review aims to identify, describe and critically evaluate longitudinal studies in the field. Method: Five electronic databases were systematically searched from January 2000 to February 2020. Peer-reviewed articles assessing gender identity and psychosocial outcomes for children and young people (<18 years) with gender diverse identification were included. Results: Nine articles from seven longitudinal studies were identified. The majority were assessed as being of moderate quality. Four studies were undertaken in the Netherlands, two in North America and one in the UK. The majority of studies had small samples, with only two studies including more than 100 participants and attrition was moderate to high, due to participants lost to follow-up. Outcomes of interest focused predominantly on gender identity over time and emotional and behavioural functioning. Conclusions: Larger scale and higher quality longitudinal research on gender identity development in children is needed. Some externally funded longitudinal studies are currently in progress internationally. Findings from these studies will enhance understanding of outcomes over time in relation to gender identity development in children and young people.