"Tests of Aptitude for Language Learning" (TALL) is an openly accessible internet-based battery to measure the multifaceted construct of foreign language aptitude, using language domain-specific instruments and L1-sensitive instructions and stimuli. This brief report introduces the components of this theory-informed battery and methodological considerations for developing it into an open research instrument. It also presents the preliminary results from the initial validation of TALL carried out on data collected from Chinese L1 participants (n = 165) from a university setting who took two rounds of tests (with counterbalanced test items) with a minimum 30-day interval. The results of data analyses at subtest, item, and battery levels suggest that, in general, TALL has satisfactory reliability and can be used to measure aptitude conceptualized in the theoretical frameworks on which it has been developed. This report also highlights the value of TALL as a convenient data collection tool openly accessible to any researcher for free, its potential for facilitating an open data pool for high-quality syntheses of aptitude-related research findings, and its implications for Open Research practices in testing language-related constructs.