The 30-year-old, who can play at right or left back, joined Napoli for 8.5 million euros ($9.38 million) from Siena in 2009 and played 163 games for the Serie A side, including a four-year spell under Watford manager Walter Mazzarri.
He started his professional career in Colombia with Atletico Nacional, where he won three league titles, and played nine times on loan at Bologna during the second half of last season.
Zuniga has made 62 appearances for Colombia, including four games at the 2014 World Cup in Brazil.
($1 = 0.9065 euros)
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(Writing by Ken Ferris; Editing by Sudipto Ganguly)
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