Dom Luís I (31 October 1838 in Lisbon – 19 October 1889 in Cascais) was a member of the House of Braganza, and King of Portugal and the Algarves from 1861 to 1889. The second son of Maria II and Ferdinand II, he acceded to the throne upon the death of his brother Pedro.

Luís was a cultured man who wrote vernacular poetry, but had no distinguishing gifts in the political field into which he was thrust by the deaths of his brothers Pedro V and Fernando in 1861. Luís's domestic reign was a tedious and ineffective series of transitional governments called Rotativism formed at various times by the Progressistas (Liberals) and the Regeneradores (Conservatives – the party generally favoured by King Luís, who secured their long term in office after 1881). Despite a flirtation with the Spanish succession prior to the Franco-Prussian War of 1870–71, Luís's reign was otherwise one of domestic stagnation as Portugal fell ever further behind the nations of western Europe in terms of public education, political stability, technological progress and economic prosperity. In colonial affairs, Delagoa Bay was confirmed as a Portuguese possession in 1875, whilst Belgian activities in the Congo (1880s) and a British Ultimatum in 1890 denied Portugal a land link between Portuguese Angola and Portuguese Mozambique at the peak of the Scramble for Africa.

Luís was mostly a man of the sciences, with a passion for oceanography. He invested a large portion of his fortune in funding research boats to collect specimens in the oceans of the world, and was responsible for the establishment of one of the world's first aquariums, the Aquário Vasco da Gama in Lisbon, which is still open to the public with its vast collection of maritime life forms, including a 10 meter long squid. His love for sciences and things new was passed to his two sons.

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