Customers of the bank were unable to access current accounts, savings and loans services since Tuesday due to a computer glitch. Branch-based services and card payments were not affected.
Around 300,000 of its seven million customers were affected the outage, which has now been resolved.
"Customers can now see their accounts online as normal," the bank said on its official Twitter account.
"We apologise for any inconvenience caused and thank customers for their patience."
Tesco Bank moved to Fiserv's Signature core banking platform prior to migrating off Royal Bank Scotland's systems in 2011. The separation process led to widespread disruption to services, leading to parent company Tesco slowing its integration programme to avoid further technical problems.
IT failures have become a common occurrence for large UK banks which rely on outdated technology and complex architectures built up over many years.
This has led to UK watchdog the Financial Conduct Authority to investigate the legacy infrastructure underpinning banking services. It has also handed out a £56 million fine to RBS following an outage in 2012 which affected millions of customers over three weeks.
Smaller lenders such as Sainsbury's Bank have also been affected in recent months too, while the UK's largest building society Nationwide has experienced IT problems despite recently moving to a SAP core banking solution.