The group, which also owns ad agency Karmarama and production firm Kream, did not disclose financial details of the deal.
Grape's nine staff will move into the group's offices in Farringdon on 10 June. They will join Kaper's 12 staff in a new entity, the name of which is yet to be finalised.
The management team will be headed by Kaper founder and former MySpace comms director Chris McCafferty.
The two agencies had already been working closely on a number of joint clients and new business opportunities.
Grape was founded four years ago by Mark Malone. Creative director Joe Holt joined a year later from Albion to form the management team.
It works on social media for clients including LinkedIn, Tesco and Samsung.
McCafferty said the acquisition would help strengthen Kaper's social media capabilities. Kaper's clients include Unilever, Time Out, Maille and MasterCard.
Kaper won PRWeek's New Consultancy of the Year Award in 2011.
