Dos Santos’ move from Spain’s Villarreal had been rumored for several days.
The official announcement came hours after the Galaxy announced they had sacked coach Curt Onalfo and replaced him with former Galaxy coach Sigi Schmid.
Jonathan Dos Santos will be reunited with his older brother, Giovani Dos Santos, who arrived at the Galaxy in July 2015, also from Villarreal.
The signing of Jonathan Dos Santos strengthens a Galaxy central midfield in flux during 2017 with new arrivals Joao Pedro, Jermaine Jones and local product Rafa Garcia carrying most of the minutes.
Recently, left back Ashley Cole has also filled in at the position.
Since an off-season makeover the Galaxy have struggled in 2017. They are in ninth place in the Western Conference, five points from the sixth and final spot in the MLS Cup playoffs.
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