The Hornets defender joins compatriot Jesus Corona at the Primeira Liga runners-up after agreeing to spend the 2015-16 campaign on loan in Portugal
Porto have signed Mexico international full-back Miguel Layun from Premier League newcomers Watford.
The deal was confirmed late on Monday and although details of the transfer were not disclosed, the defender is expected to move on loan until the end of the season.
Layun follows in the footsteps of Mexico team-mate Jesus Corona, who joined the Portuguese giants from FC Twente on the same day.
The 27-year-old Layun appeared in three of Watford’s four Premier League matches this term, scoring in their 2-2 draw with Everton on the opening day.
Layun arrived at Watford from CONCACAF Champions League winners America in January this year, signing a four-and-a-half year deal.
He played 17 league games in his first season as Watford gained promotion to the Premier League.