The robot - which was created by a team from SparkFun Electronics - was able to open a safe in less than half an hour by reducing the amount of combinations possible from one million to just 1,000.
The robot cracked the lock during a demonstration on stage in Las Vegas at Def Con.
SparkFun's Nathan Siedle said: "That was one of the scariest things we've done. Lots of things can go wrong, and this was a very big audience. We're really happy it opened up.
"We designed it for a particular type of safe, but it doesn't really matter - you can actually 3D-print a coupler that can match any safe that you may have. No matter how much money you spend on a safe ... nothing is impervious."
The robot works by using the size of indents on the dial to detect and crack the first of three dials, reducing the number combinations significantly. After that, the robot is able to test the 1000 combinations easily.
The team decided to create the safe when Mr Siedle's wife Alicia bought her husband a second-hand safe on eBay. It was offered cheap because the seller didn't know the code.
Explaining about how it came about, they wrote on their blog: "My wife is amazing; for Christmas this year, she gave me a fire safe she purchased off of Craigslist. It was super cheap because the seller didn't have the combination. Best present ever. We were able to crack our Craigslist safe in 40 minutes and 42 seconds!
"The SparkFun Safe Cracker is a variant of the Autodialer. Instead of attempting every combination in the solution domain (called brute forcing), we use some tricks to reduce the domain and shortcuts to speed up the testing process. In addition, the SparkFun Safe Cracker is cheap (~$200), magnetically adheres to the safe, and is non-destructive; when we're done you'll never know we were there."