Donovan spent a brief period with the English Premier League club two seasons ago and will return for a second stint after Major League Soccer (MLS) and his American team agreed to release him.
The 29-year-old is likely to be available for Everton's home game with Bolton Wanderers on January 4. His final game could be the Merseyside derby at Liverpool on February 25.
"The opportunity to return to Everton and play for such a well-respected club and a manager I hold in such high regard was something that was simply too good to pass up," Donovan said in a statement.
Everton manager David Moyes was equally pleased to set up the deal.
"I am delighted we have managed to get Landon back," Moyes told his club's website. "He will give us some good experience over January and February.
"He did well for us two years ago and hopefully will return with those same qualities. His season has just finished and like last time we need him to hit the ground running because the games come thick and fast at this time of year."
Last month Donovan teamed up with former England captain David Beckham to help the Galaxy win the MLS championship, scoring the winning goal in the final against Houston.
As part of the loan deal Donovan will return to the Galaxy early next year in time for the CONCACAF Champions League quarter-finals and MLS season-opener.
