WCRS won the creative business after a competitive pitch against Leagas Delaney and the marketing communications agency Table 19.
The Engine-owned shop will be charged with growing the brand's awareness and building its customer base. It will work on Safestore's next campaign, which is expected to break during the summer and will be supported by digital activity.
Media planning will be handled by Engine's strategy department, while the7stars will be responsible for media buying.
David Cox, the head of sales and marketing at Safestore, said: "We saw an extremely strong set of pitch proposals, but the strategic rigour and creative flair of WCRS really stood out. This is an important time for the Safestore brand and, in WCRS, we've found a partner we think well-equipped to help us exceed our big ambitions."
Rick Hirst, the client services director at WCRS, said: "Working with Safestore is one of those rare opportunities - a market-leading business with a great product that wants brand fame and has a clear vision for what they want to achieve."
Safestore, which offers personal and business storage, was founded in 1998 and has grown to more than 90 stores across the country.