The 22-year-old has signed a contract until 2016 with the Bundesliga 2 outfit.
Leckie made a total of 28 appearances and scored four goals in FSV's record-breaking season, earning his second cap for the 'Socceroo's' as a result of his fine form.
"Mathew showed last season that he can really improve our side," said Frankfurt's executive director for sport Uwe Stöver. "That is why we are delighted that he has chosen to continue along this path with FSV."
'The right step'
Leckie joined Frankfurt at the start of last season from the Foals, and he has never for one second regretted dropping down a division. "We played a fantastic season with FSV and I also see it as the right step for my personal development to carry on working with this team and with this coaching team," said the Australian. "We want to latch onto this success."