The 28-year-old has made just seven first team appearances for the English Premier League side since arriving from Monaco in August 2011 which has stagnated his international prospects.
Park, who has scored 23 goals in 61 internationals, will hope the increased playing opportunities at Watford can help him force his way back into the Hong's squad ahead of the World Cup in Brazil later this year.
"I feel much relieved that he can make a fresh start," Hong told Yonhap news agency from Los Angeles where he is holding a 23-man pre-World Cup training camp.
"Park will have to show a good performance in the coming matches."
Last season, Park was loaned to Celta Vigo in La Liga where he became the first South Korean to score in Spain's top flight.
"Everyone at Arsenal wishes Ju the best of luck for his time with Watford," the Premier League side said, adding the move was subject to completion of standard regulatory processes.