Athenahealth Inc., which provides electronic record and billing systems to hospitals and other medical industry clients, has agreed to buy the Arsenal on the Charles office campus in Watertown, the company said Wednesday.

Athenahealth will pay $168.5 million to acquire the 29-acre parcel from Harvard University, which is expanding in nearby Allston. The campus already serves as Athenahealth’s headquarters, but executives said buying the property will allow them to expand the business in coming years.