In a statement, UCSF said it had initially detected a security incident at its School of Medicine on 1 June.
Several IT systems were quarantined and the infection was isolated from the core network. But the ransomware encrypted a number of services leading to negotiations with the criminals led by an unspecified cyber security consultant.
"Our investigation is ongoing but, at this time, we believe that the malware encrypted our servers opportunistically, with no particular area being targeted," UCSF said.
Australian customer experience firm Stellar, that also operates across Asia, North America and Africa, took a hit from the NetWalker ransomware in May.
The UCSF said it had paid the ransom because "the data that was encrypted is important to some of the academic work we pursue as a university serving the public good".
"We therefore made the difficult decision to pay some portion of the ransom, approximately US$1.14 million, to the individuals behind the malware attack in exchange for a tool to unlock the encrypted data and the return of the data they obtained.
"This incident reflects the growing use of malware by cyber criminals around the world seeking monetary gain, including several recent attacks on institutions of higher education.
"We continue to cooperate with law enforcement, and we appreciate everyone’s understanding that we are limited in what we can share while we continue with our investigation."
The BBC reported that it had was able to witness the negotiations for the ransom first-hand.