Liuzzi was on the market after losing his Force India seat at the end of 2010. He tested for HRT at Catalunya last month in what the team openly admitted was an audition for the race drive, and has now completed his deal.
"I am really happy to have signed this agreement with Hispania Racing," said Liuzzi.
Kolles believes the experienced Italian will bring a great deal to HRT as it tries to get closer to the established teams' pace.
"I have known Tonio for many years as he raced with me in lower categories," said Kolles. "He is a very professional driver who will bring a lot of benefits to the team.
The team also confirmed that it will unveil its new F111 on Friday at Barcelona.