The Premier League club will have an option to buy the Czech Republic international, who passed a medical with the club on Sunday.
Baros joined Lyon from Aston Villa last January, but has failed to break into the team at the French Ligue 1 side.
Portsmouth boss Harry Redknapp said: “It’s a terrific opportunity to have a look at him.
“It’s a loan deal until the end of the season and then we’ll see how it goes from there.
“He’s a good player who’s played at Liverpool and Aston Villa – and he’s been the golden boot winner at a European Championships.
“He’s got pace and he makes good runs.”
