The 26-year-old joins the A-League after a successful stint at Bosnian side FK Zeljeznicar, with whom he won a League and Cup double last season.
Coach John Aloisi said Gerhardt's winning mentality and versatility will be a key asset for the Heart.
"Patrick will bring a lot to the club," Aloisi said.
"Defensive midfield is the position we see him most suited to but he's versatile. He can win the ball but he can also play the ball out of the back.
"We've had good feedback from the people we know in Europe, and they've said not only is he a good player but a good person, and that's an important factor for our club."
Gerhardt, who was born in Switzerland and is a product of the Young Boys of Bern, made his international debut for Liberia in March last year and recently featured in their 2014 World Cup qualifier against Senegal.
The midfielder trained in Melbourne last week before completing a medical.
"I'm happy to be here," Gerhardt said.
"I've seen a lot of nice things here at Melbourne Heart FC and in the city itself, so I'm confident I'll be happy here in Melbourne.
"I had a good experience in Bosnia, we had a great team and enjoyed a fantastic season, losing only a couple of games, but I'm excited by the challenge ahead in Australia."