BackgroundNature prescribing – a written recommendation by a health professional for a person to spend more time in nature for health reasons – is being heralded by some as a potential solution to loneliness, defined as a felt deprivation of meaningful connection, companionship, and camaraderie. However, such recommendations currently lack evidence on how much time in natural environments is needed to tackle loneliness and over what timescale. MethodsGeneral, emotional, and social loneliness were measured using De Jong Gierveld's 6-item scale in a nationally representative sample of 3043 Australian participants at baseline (October 2020), with 2421 and 2123 completing follow-ups at 4 months (February 2021) and 16 months (February 2022), respectively. Multilevel logistic regressions examined the odds of change in loneliness status between baseline and follow-up in relation to hours spent in nature in the 7 days prior to the baseline survey, adjusting for confounding factors that influence time spent in nature and the risk of feeling lonely. FindingsRelief from general loneliness in those experiencing it at baseline was 22.4% at month 4 and 29.4% at month 16. The equivalent findings for relief from social loneliness and emotional loneliness were 21.2% and 24.6%, and 34.2% and 41.2% at months 4 and 16, respectively. Incidence of loneliness at 4 months was 21.8% (general), 26.1% (social), and 11.8% (emotional), and at 16 months was 22.7% (general), 25.6% (social), and 12% (emotional). At each time point, the prevalence of loneliness was lower among participants with more time in nature. Just 1–2 h per week in nature (versus <1) was associated with relief from social loneliness at 4 months (Odds Ratio [OR] 1.69, 95%CI 1.10, 2.65) and at 16 months (OR 2.10, 95%CI 1.34, 3.30). Higher odds of relief from emotional loneliness were observed at 3–4 h spent in nature at 4 months (OR 1.84, 95%CI 1.11, 3.06) and at 16 months (OR 1.82, 95%CI 1.09, 3.07). Incidence of loneliness was not associated with nature contact. InterpretationIncreasing time in nature may an effective non-medical prescription for people who are lonely to find durable relief within a relatively short space of time. Findings from this longitudinal study can inform further investigation through randomised trials of co-designed nature prescriptions targeting relief from loneliness in vulnerable populations.