The 24-year-old has joined for an undisclosed fee after passing a medical on Monday afternoon and the Swans are hoping he will be cleared by the Premier League to line up against Fulham on Tuesday.
Ngog joined Bolton from Liverpool in 2011 and scored three goals in his final four appearances for the club but his contract was due to expire in the summer and he sat out Saturday's FA Cup loss to Cardiff.
Swansea are currently without last season's top scorer Michu due to an ankle problem, and Wilfried Bony and loanee Alvaro Vazquez were their only fit forwards until the Ngog deal was finalised.
Bolton, who lie 18th in the Championship, have been looking to cut their wage bill as they continue to adjust to life in the second tier.
Ngog has played for the France U21 side and, having joined Liverpool from Paris Saint-Germain in 2008, he went on to play 94 times for the Reds, scoring 19 goals.
He moved on to Bolton for £4m in August 2011 but was unable to prevent the Trotters from being relegated at the end of that campaign.
"It's a delight for me," said Ngog. "I think Swansea are a very good club. They have shown over the last couple of years that they are a good Premier League side.
"I want to help the team win games and improve in the Premier League. If I can bring my quality to the team, I will be delighted.
"Swansea are a team who like to keep the ball and play good football. I will try to introduce myself to the team and to adapt.
"The manager Michael Laudrup was obviously a top player and as a manager he has proven he is very good as well. It will be really interesting to work with him.
"Overall, it's a really good opportunity for me, and I am glad to be here."