Tunez, 33, has followed Thailand national team and SCG Muang Thong United midfielder Sarach Yooyen to the Pathum Thani-based club, who have just moved back to the top flight after spending a year in the second division.
The news of Tunez joining BG Pathum has come as a big surprise.
The veteran Venezuelan defender parted ways with former Thai League 1 champions Buriram last month after spending almost six years at the northeastern club.
Tunez, it was reported at that time, was interested in joining a Liga 2 club in Spain, but has ended up signing for another top-flight club in Thailand instead.
"I am here to win the Thai League title," Tunez said on Wednesday. "I have faced BG Pathum in the past as an opponent and know how strong they are.
"Although they have just been promoted back to Thai League 1, I think they are capable enough to win the title and I will help them achieve their target."
BG Pathum and Port are joint third in the league, two points behid leading duo Bangkok United and Ratchaburi.
Meanwhile, a top Muang Thong official has played down the impact of recent departures of some of the big names from the club.
The Kirins this month lost key defender Adisorn Promrak to Port and their captain Sarach soon joined BG Pathum.
Muang Thong director Ronnarit Suewaja said the exit of first-team stars will give chances to young players from the club's academy.
"We remain committed to winning the league title again and we plan to do this with a bunch young players," added Ronnarit.
Thai League 1, halted in early March due to the Covid-19 outbreak, is scheduled to restart on Sept 12.
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