Michail Antonio put West Ham ahead in the 23rd minute with a right-foot shot after Winston Reid headed on a corner.
Giannelli Imbula equalised with a low strike from outside the penalty area and Diouf struck two minutes from time with a fine header to lift Stoke to ninth in the standings.
Defeat ended West Ham's hopes of a top-six finish.
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(Reporting by Ed Osmond; Editing by Toby Davis)
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