Kouyate ended his four-year stay at the London Stadium outfit after signing a contract that ties him down at Selhurst Park until 2022.
The Teranga Lions skipper becomes Palace's second signing of this summer transfer window after goalkeeper Vicente Guait.
Souare has welcomed his compatriot to Roy Hodgson's team while DR Congo international Masuaku is already feeling the absence of the 28-year-old in the Hammers' fold.
"My brother, there is nothing more to say. I miss you already, thanks for everything. See you very soon," Masuaku tweeted.
"Welcome my brother," Souare tweeted.