The 27-year-old has made the move from Udinese, though his entire six-year stint on the Serie A club's books has been spent on loan at Spanish side Granada. Both those clubs, like Watford, are owned by the Pozzo family.
The French-born player has represented his adopted nation since 2011 and played in all three games of their miserable 2014 World Cup campaign.
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Nyom began his career with Arles-Avignon in France before joining Udinese - and Granada - in 2009, going on to make over 200 appearances in Spain.
He is Watford's seventh signing of a busy summer ahead of their return to the top flight, following goalkeeper Giedrius Arlauskis, fellow defenders Sebastian Prodl and Jose Holebas, midfielders Valon Behrami and Etienne Capoue and the permanent capture of forward Matej Vydra.
Nyom will join up with his new team-mates on the Hornets' pre-season training camp in Germany, where they will take on SC Verl, Eintracht Braunschweig and Paderborn on Wednesday, Friday and Sunday.
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