Bitstamp resumed operations Friday after an attack earlier in the week resulted in the loss of 19,000 bitcoins and forced the exchange to close.
"Our team has been working day and night to rebuild and restore security to the Bitstamp site so customers can resume transacting with us quickly, safely, and confidently," wrote CEO Nejc Kodri in a blog post on Friday announcing the return of the service.
During the shutdown, which began Monday, the company rebuilt its website, deployed new hardware, migrated its operations to Amazon Web Services and implemented multisignature transactions, a security measure designed to make stealing bitcoins more difficult.
"While this decision means we have not been able to provide you with services for a number of days, we feel this extra measure of precaution was in the best interest of our customers," Kodri wrote.
Hackers made off with $5 million and targeted operational or hot wallets, which allow people to conduct transactions instantly.
During the incident, Bitstamp reassured customers that their bitcoins were safe and that it could cover the lost virtual currency. The company said most of its currency is kept in storage systems not connected to the Internet. Other exchanges have folded after hackers drained their accounts.