This article presents and reviews a new methodological resource for research in second language acquisition (SLA), CEDEL2 ( Corpus Escrito del Español L2 ‘L2 Spanish Written Corpus’), and its free online search-engine interface ( cedel2.learnercorpora.com ). CEDEL2 is a multi-first-language corpus (Spanish, English, German, Dutch, Portuguese, Italian, French, Greek, Russian, Japanese, Chinese, and Arabic) of L2 Spanish learners at all proficiency levels. It additionally contains several native control subcorpora (English, Portuguese, Greek, Japanese, and Arabic). Its latest release (version 2) holds material from around 4,400 speakers, which amounts to over 1,100,000 words. CEDEL2 follows strict corpus-design criteria (Sinclair, 2005) and L2 corpus-design recommendations (Tracy-Ventura and Paquot, 2021), and all subcorpora are equally designed to be fully contrastable, as recommended by Contrastive Interlanguage Analysis (Granger, 2015). Thanks to its design and web interface, CEDEL2 allows for complex searches which can be further narrowed down according to its SLA-motivated variables, e.g. first language (L1), proficiency level, self-reported proficiency level, age of onset to the L2, length of exposure to the L2, length of residence in a Spanish-speaking country, knowledge of other foreign languages, type of task, etc. These CEDEL2 features allow L2 researchers to address SLA questions and hypotheses.