The Financial Conduct Authority said it had fined Santander, the eurozone's biggest lender which has branches in Britain, the equivalent of $20.5 million or €14.9 million, "for widespread investment advice failings."
Tracey McDermott, director of enforcement and financial crime at the FCA, said Santander had "let its customers down badly".
She added in a statement: "Customers trusted Santander to help them manage their money wisely, but it failed to live up to that responsibility.
"If trust in financial services is going to be restored, which it must be, then customers need to be confident that those advising them understand, and are driven by, what they need."
Steve Pateman, head of banking at Santander UK, said the lender regretted that "investment sales processes did not meet the required regulatory standards".