Diop, 21, joins from French side Toulouse on a five-year-deal, with the Hammers having beaten Everton, Borussia Dortmund and Sevilla to the £22million deal. Fabianski is expected to compete with Adrian for the goalkeeper's position.
West Ham United are pushing to agree terms on a £35million deal for Lazio's Felipe Anderson after landing defensive target Issa Diop on Tuesday.
Fabianski, who moved to Wales from Arsenal in 2014, told Swansea he wanted to leave the Liberty Stadium after their relegation to the Championship.
The 33-year-old is now away with Poland for their World Cup campaign and was on bench for their 2-1 defeat to Senegal yesterday.More news: Senegal and Japan fans tidy up after their World Cup matches
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"I am a very ambitious player and West Ham is a very ambitious club, as we've seen with the appointment of the new coach".
"Lukasz is an experienced and established Premier League goalkeeper, with great quality that he has shown at both domestic and global level", said West Ham's new director of football Mario Husillos.
It comes after West Ham announced their third signing after appointing Manuel Pellegrini as their new manager with the confirmation that goalkeeper Lukasz Fabianski has been acquired from relegated Swansea City for £7m on a three-year contract.
The Hammers have agreed a fee of around £7m for the Poland global, who will fill the void left by Joe Hart after he returned to parent club Manchester City.
"Of course, at this moment, Lukasz is fully focused on the World Cup finals with Poland".