The wing is linking up again with Connacht coach Andy Friend, who recruited him into the Australia sevens’ programme in 2015.
“I obviously know Andy. Playing Champions Cup Rugby in the Sportsground next season is an opportunity I just couldn’t turn down,” said Porch.
“I’m looking forward to getting over to Ireland and linking up with the squad.”
In his first nine appearances for Australia, Porch scored 96 points and earned a place at the Rio Olympic Games in 2016.
Connacht head coach Friend said: “John was a hugely impressive player at sevens level, and I believe that he will suit the style of play and culture in Connacht as we prepare for the season ahead.”