Porto's incredible 55-match unbeaten streak in Liga Sagres ended on Sunday as they were defeated 3-1 at Gil Vicente.
The reigning Portuguese champions last suffered defeat in the league on February 28, 2010, when Sporting Lisbon put them to the sword.
Sporting are one of the big teams in Portugal but tiny Gil Vicente, who have only finished in the top 10 in the top-flight three times in their 88-year-history, were not expected to be the team that ended Porto's golden run.
The warning signs were evident at half-time though, with the hosts leading 2-0 thanks to a pair of Claudio strikes.
The second was from the penalty spot after a Nicolas Otamendi foul and Porto boss Vitor Pereira attempted to change things up with the introduction of Danilo and Fernando Belluschi for the second half.
But Porto looked destined to lose when Andre Cunha gave Gil Vicente an amazing 3-0 lead just seven minutes after the break.
Silvestre Varela mustered a response for Porto but with just 13 minutes left to play, it was too little too late.
Porto's defeat meant that they failed to equal the record unbeaten streak in the league, set by rivals Benfica at 56 matches, recorded between 1976 and 1978.
In other results, American defender Oguchi Onyewu scored a double as Sporting Lisbon defeated Beira Mar 2-0.
Olhanense came from two goals behind to defeat Vitoria Setubal 3-2 and Manuel Jose and Michel both bagged a brace in Pacos de Ferreira's 4-2 win at Uniao De Leiria.