The 25-year-old, who can play at right-back as well as his preferred central defensive position, has attracted the interest of a number of Barclays Premier League suitors.
Arsenal scouts were in attendance for the Hornets' FA Cup fourth round defeat to Tottenham Hotspur on Friday evening, a game in which Mariappa impressed.
After initially insisting he would not make any transfer window signings, Wenger is now ready to reinforce his defensive department at the Emirates Stadium.
The Gunners' frailty at the back was on show again in yesterday's 3-2 victory over Aston Villa, with the north Londoners forced to come back from two goals down to ensure progression into the fifth round.
Inconsistent performances from a number of Arsenal defenders, particularly Johan Djourou in recent weeks now looks to have forced the arm of Wenger, who will look to tie up a deal imminently.