The article analyses co-authorship between Brazilian and Portuguese authors in the periods 1980-90 and 2005-15. According to the Web of Science, between 1980 and 1990, Portugal published 221,229 documents (articles, proceeding papers and reviews) and Brazil published 656,866 documents. However, taking into account the Category Normalized Citation Impact, which is the impact of standardised citations for the category, Brazil reached 0.76 (below world average) and Portugal 1.08 (above the world average). The article analyses the number of documents, knowledge areas, journals in which they were published, and Portuguese and Brazilian institutions involved while considering the investments that each country made in science and technology. When the two periods were compared, the number of published documents increased, and the areas of knowledge expanded considerably. The number of unique journals in which documents were published also increased as well as institutions that published in co-authorship. The "hard" sciences excelled in co-authored publications. The financing of science and technology and research and development in the two countries has increased considerably since the 1990s, declining in Portugal after 2008 due to the international financial crisis and declining in Brazil from 2015. However, international cooperation and co-authorship is fundamental to the advancement of scientific knowledge and the quality and visibility of scientific production.