The California-based company said hackers hit its Horizon "bridge," one of its key products, which transfers crypto between different blockchains - the underlying software used by digital tokens such as bitcoin and ether.
Harmony tweeted late Thursday that it was "working with national authorities and forensic specialists to identify the culprit and retrieve the stolen funds."
Harmony did not immediately respond to requests for comment.
Thefts have long plagued companies in the crypto space, with blockchain bridges increasingly targeted.
So far this year, more than $1 billion has been stolen from bridges, according to blockchain analytics firm Elliptic.
In March, hackers stole around $615 million worth of cryptocurrency from Ronin Bridge, used to transfer crypto in and out of the game Axie Infinity. The United States linked North Korean hackers to the theft, one of the biggest crypto heists ever.
