The new client win for Thought Machine continues a recent run of wins which includes Wise, TransferGo and JP Morgan.
Thought Machine has signed up a Polish bank with over four million retail customers to its cloud-based core banking services platform.
Vault is Thought Machine's cloud-based platform that contains a range of banking services, including checking accounts, savings accounts, loans, credit cards and mortgages.
Last month, new investors also joined Thought Machine's cap table with a $200m Series C funding round that pushed the cloud banking provider's valuation to more than $1bn.
Led by New York VC firm Nyca Partners, other new investors included investment banks Standard Chartered, ING and JP Morgan Chase.
ING Bank Śląski has around 4.3m retail clients and 509,00 corporate clients with 262 bank outlets.
The Polish bank is using Vault to "modernise systems, launch new products and enhance customer experience" says Thought Machine.
"The banks that choose to embrace this will deliver on ever-evolving customer needs, and win in an increasingly competitive market. ING Bank Śląski, Poland's leading financial institution, is one of these game-changers.
"We look forward to working on this core transformation programme and for ING Bank Śląski to deliver material value for its customers."
Thought Machine, founded in 2014, employs more than 500 people across offices in London, New York, Singapore, Sydney, and Melbourne.