The Russia midfielder made just 16 league appearances for the club last season, most as substitute, having returned from a loan at Portsmouth.
"The last two months of last season at Chelsea were painful for me because I'm not used to sitting on the bench," he told the BBC Russian Service website.
Addicks boss Alan Curbishley said: "I would like to thank Jose Mourinho for allowing Alexey to join us."
Smertin admitted he had received other offers, but said he wanted to stay in London.
Smertin's decision to go to Charlton will be a disappointment to Portsmouth, who had the player on a season-long loan in the 2003-04 season.
"I decided I needed match practice week in and week out. That will be better for me and for the Russia national team," he said.
"No training, regardless of its quality - and Chelsea training was of the highest quality - can compare with playing competitively."
But there is still a chance that Smertin could appear in a Chelsea shirt again before the end of the season.
"When I told Jose Mourinho I wanted to leave he was surprised," Smertin admitted.
"He told me he still counts on me and that Chelsea have a long season in front of them, and we decided that if something goes wrong for me at Charlton I will be able to rejoin the Blues.
"He said that they would be waiting for me."
Smertin was Russian footballer of the year in 1999 and represented his country in Euro 2004, as well as the 2002 World Cup.