The French defender, who played 63 games for the Hammers in his two seasons at Upton Park, will complete the switch to Fratton park on the successful completion of a medical.
Schemmel critisised West Ham's ambition earlier this month, making his departure from Upton Park almost inevitable.
"I'm more than happy to take him on," Pompey boss Harry Redknapp told the club's official website.
"He will definitely do for me as I don't mind the so-called bad boys because they keep you on your toes and make life interesting."