The personal information included names, addresses and social security numbers.
State Street said there was no evidence the data had been misused, but it declined to say if the stolen equipment had been recovered. It is working with federal and local law enforcement agencies on the matter.
The company, a Boston-based provider of financial services to institutional investors, said 5,500 employees and 40,000 customers of Investors Financial Services, which it acquired last year, were affected.
The computer equipment was stolen from a vendor hired by Investors Financial Services to provide legal support services.
State Street said it has begun alerting affected customers and employees and that it would provide them with access to credit-monitoring services at no cost.
The theft occurred in December. State Street said it was informed of the theft in January and needed the past four months to analyze what and whose information had been stolen. That analysis was completed Wednesday.
State Street said this was the first case of data theft in its history.