Introduction: Valid and reliable tools for measuring obesity-related behaviours in young children that are brief and can be administered quickly and cost-effectively in large-scale population studies are needed. The objectives of this systematic review were to describe brief tools that measure dietary intake, physical activity, sedentary behaviour, and sleep in young children. Methods: A systematic review of studies published in English in six databases (CINAHL, Medline, Embase, PsycINFO, HaPI, and Cochrane) prior to April 2018 was undertaken using the PROSPERO protocol and PRISMA guidelines. Included studies were those reporting the psychometric properties of brief (≤15 items) tools that measure dietary, activity, or sleep-related behaviours, alone or in combination, in children birth to 4.9 years of age. Results: The search identified 11 379 papers, 200 full-text articles were screened for eligibility, and 12 met the inclusion criteria. Three studies measured two behavioural domains, while most assessed a single behaviour (three diet, five physical activity, one sleep, and none sedentary behaviour). Only two (one diet, one sleep) focused on the under 2 age group. Few studies assessed reliability, and validity and findings were mixed. Conclusions: There is a need to develop brief tools to measure early life obesity-related behaviours, particularly those assessing sedentary behaviour and sleep and tools that cover multiple domains.