The 24-year-old former Hansa Rostock, SK Sturm Graz and SV Werder Bremen campaigner has appeared 42 times in the Bundesliga 2 (scoring five goals) and 63 times in the third division (two goals), while he also played 20 Austrian Bundesliga matches.
"We're delighted with Timo's signing because he came on leaps and bounds in the second half of last season," said Braunschweig's sporting director Marc Arnold. "He's a versatile player and a further alternative for our coach on the left."
Perthel added: "Eintracht was my first choice and it's a great situation to be able to prove myself in the Bundesliga. I'm looking forward to pre-season and my time in Braunschweig."
Henn pens deal
He was welcomed by around 3,000 fans on the club's first day of pre-season training in preparation for their first Bundesliga campaign in 29 years. The congregated fans were also given the news that Matthias Henn had extended his contract by a year.
"We know what he gives us and he will maintain a strong battle for places in the centre of our defence," said Arnold of the defender who made three appearances last season after returning from a one-year injury absence. "He's always given it his all and fought his way back after injuries."