The 23-year-old striker is contracted with Napoli until 2014 after arriving at the Serie A club last year, and the Bundesliga deal will not include a buy option.
Hoffer, who has won 16 caps for Austria, is hoping to break into the first team at the Fritz Walter Stadion.
"Kaiserslautern is a good club with a great atmosphere," Hoffer told Austrian newspaper Niederosterreichische Nachrichten.
"They were looking to find a striker who is able to break quickly into areas.
"I am looking forward to picking up first-team experience at the club."