Luis I (English: Louis), the Popular (Port. o Popular) - (Lisbon, October 31, 1838 – Cascais, October 19 1889) was the 32nd (or 33rd according to some historians) King of Portugal between 1861 and 1889. He was the second son of Maria II da Glória and Ferdinand of Saxe-Coburg-Gotha.

Luis was a cultured man who wrote vernacular poetry, but otherwise had no distinguishing gifts in the political field into which he was thrust by the deaths of his brothers Pedro V and Ferdinand in 1861. Luis' domestic reign was a tedious and ineffective series of transitional governments formed at various times by the Progressives (Liberals) and Regenerators (Conservatives – the party generally favoured by King Luis, 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, Luis'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, Delgoa Bay was confirmed as a Portuguese possession in 1875, whilst Belgian activities in the Congo (1880s) denied Portugal a land link between Angola and Mozambique at the peak of the Scramble for Africa.

Luis was mostly a man of the sciences, with a passion for oceanography. He invested great amounts of his fortune in funding research boats to collect specimens in the oceans of the world. He was responsible for the establishment of one of the World's first Aquariums, 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.