The 35-year-old was last week given permission to negotiate with other clubs after being told by Flamengo coach Dorival Junior that he was no longer a part of the Rio outfits plans, reports Xinhua.
"This move is something that I have been wanting for a while and finally it has materialised," Liedson said on his official website Thursday.
"I am grateful to Porto for giving me this opportunity and hope to be able to repay their faith in the best way possible."
Liedson, who has won 14 international caps for Portugal, endured a frustrating debut season with Flamengo last year, scoring just four goals from 17 appearances.
It will be his second spell in Portugal's top flight, having played 214 matches and scored 116 goals for Sporting Lisbon from 2003 to 2011.