The 30-year-old United States international spent five-and-a-half seasons at Craven Cottage before leaving them to join Tottenham on a three-year contract in August 2012.
However he spent only one season at White Hart Lane before heading back to the USA when he signed a four-year deal with the Sounders on August 3.
Dempsey initially joined the Cottagers from New England Revolution in January 2007 and was twice voted the club's player of the season.
He scored 60 goals in 225 appearances during his first spell with Fulham.
Dempsey's return has been mooted for several days and Fulham boss Rene Meulensteen said last week: "Clint has spent so many years playing in the Premier League.
"We know where his strengths lie whether it is coming in from the left or playing as a deeper striker or a number nine depending on who you are playing against. The most important thing is that he will be a good boost for us."
Dempsey has played 12 MLS games for Seattle so far and finished with only one goal and one assist in his first campaign with them.
But he is hoping to use his spell with Fulham during the MLS off-season to rediscover his best form ahead of the World Cup in Brazil next summer.
He told Fulham's official website: "It feels good to be back. This (Fulham) has always seemed like my home in Europe and I am excited to be back and be part of this team and play in this league.
"I wanted to get back to playing at the highest level and getting back and helping the team in the situation where they are not where they want to be in the table.
"But going into a World Cup year you want to get into that top form and carry that forward so hopefully I can have a good showing."